Have you ever looked at the tires on your car — I mean, really looked at them? There are a lot of numbers, letters, and graphics there. Aside from the usual brand markings and tire names, like Grabber, Primacy, Potenza or Raptor, there are a lot of other pieces of information molded into the sidewall of your tires. What does all this stuff mean? Why is it there and should you really care about any of it? Let’s go through all of it, and then you can decide what matters to you.
Tires are some of the most heavily regulated items on your car. In the US, vehicles are regulated by the Federal Motor Vehicle Safety Standards (FMVSS), and there are no fewer than four that pertain to tires: FMVSS 109, FMVSS 110, FMVSS 119, and FMVSS 139. Here is what they cover:
FMVSS 109 – “New Pneumatic and Certain Specialty Tires” first adopted in 1967. This was the original tire standard that all new tires had to meet.
FMVSS 110 – “Tire Selection and Rims for Motor Vehicles with a GVWR of 4,536 Kg (10,000 lbs) or Less.” This standard sets requirements for what tires and rims can be chosen for a new vehicle. Among other things, it ensures that the tires are sized properly to carry the weight of the vehicle.
FMVSS 119 – “New pneumatic tires for motor vehicles with a GVWR of more than 4,536 kilograms (10,000 pounds) and motorcycles.” Applicable to motorcycle tires and heavy truck.
FMVSS 139 – “New Pneumatic Radial Tires for Light Vehicles.” This standard is a revision of FMVSS 109 following the passage in congress of the “Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act.” This act, which became law on November 1, 2000, was in part a response to the Firestone tire issues on the Ford Explorer. In response to section 10 of this act, the National Highway Transportation Safety Administration proposed and passed FMVSS 139 to make tire safety and testing more stringent. It is very similar to FMVSS 109 but primarily raises tire testing speeds and testing lengths.
With that established, let’s talk about the markings on the side of the tires using my 2015 Mustang as an example. We’ll forget about the brand and tire model name because that’s kind of boring anyway. Let’s focus on the other stuff. Here are all the numbers and letters shown on the side of my tires:
Now let’s go through them one by one.
You probably already know how tire size works, so if you want to skip ahead, you’re welcome. For the rest of you, tire size follows the following format:
Shown in red are three digits followed by a slash, followed by two digits, followed by two letters ending in “R” or just the letter “R” by itself, then followed by two more digits.
Shown in green, the first three digits are the section width of the tire in millimeters as measured with the tire mounted on a standard width rim. Every tire size has a standard rim width defined by the relevant authority. In the U.S. that authority is the Tire and Rim Association (or T&RA) while in Europe it is European Tyre and Rim Technical Organisation (or ETRTO).
The two digits following the slash represent the aspect ratio, which is a percentage multiplier that determines the section height. Take the three-digit tire width, multiply it by this percentage and you get the distance from the rim to the tire tread.
The last two digits denote the diameter of the wheel/rim in inches.
You may also see a “P” in front of the tire size which means it is a Passenger tire. Light truck tires may have “LT” instead of the “P”.
In the case of my Mustang, the tires on this car show 265/35R20.
This means the tires are 265 mm wide, have a section height of 265 x 35% = 92.75 mm, and sit on a 20-inch diameter rim.
These numbers are also very useful if you want to know the overall diameter of your tires. It’s done by multiplying the width by the aspect ratio, times two since there are two sidewalls comprising the diameter, then adding the rim diameter converted to millimeters. For the Mustang tire it would be: 265 x 35 / 100 x 2 + (20 x 25.4) = 693.5 mm, or 27.3 inches.
It’s interesting to note that tire size is one of the only instances in the worldwide auto industry where you will see a mix of units. Width is metric but rim diameter is imperial and it’s true throughout the world. Even in countries that have used the metric system since it was invented over 200 years, rim diameter is denoted in inches. To understand this, let’s look at some history.
Mixing English And Metric Units
Before the advent of radial tires, in the days of bias ply tires (which we’ll get to in a sec), size was given by an alpha-numeric system that used a letter followed by a two-digit number, then a dash followed by a two-digit number — something like: F70-15. The letter denoted the tire load range, with A being the lowest and getting progressively higher as you move up the alphabet. The two-digit number is the aspect ratio, with higher numbers meaning a taller tire, and the number after the dash was the rim diameter in inches. Since the load capacity of a tire is a function of the size of the tire, the load range number gave a general idea of how large the tire was, but it was far from perfect. Also, what is missing from this method is anything related to the tire width. It really wasn’t a very good system.”
Before the alpha-numeric system came into use in the mid 60’s, tire sizes were denoted by a number like this: 6.50-13, where 6.50 was the tire width and 13 was the rim diameter. Both numbers were inches. Tire sizes where the first number ended in a zero almost always used a 90% aspect ratio. Later developments had tire sizes like 5.75-15 which meant the aspect ratio was 80 to 84 percent.
Since the American auto industry was very dominant in the early days of cars, everyone who wanted to sell tires in the U.S., regardless of country of origin, had to use this sizing method. So, in the late 60’s and early 70’s, when Michelin invented radial tires, they had to come up with a sizing system that would accommodate the American rim diameters, which were all in inches, but they wanted to use the metric system as much as possible. This is why we now have a tire sizing system that uses a mix of units. If you want to dive deeper into this rabbit hole, click Coker Tire’s website here.
The last set of numbers of the tire size — the one denoting the rim size (see “ZR” above) — includes either one or two letters before it. The second letter shows the tire construction and will almost always be “R” which stands for Radial. The first one, if it is included, is the speed rating of the tire and denotes how fast the tire is certified to go before you run the risk of it coming apart at the seams. If the tires on your car show only the letter “R” in this spot, then the speed rating will be part of the load rating which we will get to later. Here is a chart showing what each letter means:
Note that speed rating is a European thing. For the U.S., tires only need to meet FMVSS 139, which tests tires up to 100 mph. In Europe, where higher speeds are possible on the Autobahn, the speed rating is set using standard ECE R-30 which describes the test procedure tire manufacturers must use to certify the speed rating they show on the tire sidewall.
Phew, we made it through the tire size. Let’s move on.
Load Rating And More On The Speed Rating
The next number is the load rating. This is the two-digit (or sometimes 3 digits) number after the tire size. In the case of the Mustang, it is 99. This number shows how much load the tire can handle and again must conform to the standards described in FMVSS 139 and ECE R-30. In a tire, the load is carried by the air inside the tire. The more air in the tire, the more load it can carry.
There are two ways to get more air in a tire: inflate it to a higher pressure or make the volume of air inside the tire larger. It’s easy to increase the air pressure inside a tire but you can only go so far. Pretty soon the air pressure is so high you risk blowing the tire out. The better way is to increase the size of the space inside the tire by changing the dimensions of the tire. Think of the air inside the tire as a donut. How can we make the volume of this donut larger? Well, we could increase its width, we could increase its outside diameter or we could decrease the size of the hole in the middle. All of these have implications on other things though. Increasing the width may cause the tire to interfere with the wheelhouse; an increase in the outside diameter could cause similar issues. Decreasing the size of the hole in the middle means using a smaller wheel which may not fit the suspension or brakes. This is why these questions MUST be answered in the very early stages of a car’s design and why it’s so hard to change once the vehicle is in production. The tire size and the corresponding maximum weight and cargo loading of the vehicle are some of the first things that get decided when a new vehicle is being designed.
Let’s get back to the Mustang example. Remember that the load rating is 99, which in and of itself doesn’t mean very much; it’s just a number that corresponds to an actual load value. Here is a chart of what the numbers mean:
From this, we can see that each of my Mustang’s tires can carry a load of 1,709 lbs or 775 Kg. This means that, as far as my tires are concerned, the maximum weight my Mustang could be 4 x 1709 = 6,836 lbs. That is of course way more than the car will ever weigh, so in this example load rating is not an issue I need to worry about. But in the case of a pickup truck, it might very well become an issue when the truck is fully loaded. Depending on the truck and its tires, it could be easy to dump a load of gravel, or sand bags, or dirt in the back and overload the rear tires; this could cause a blow-out down the road.
Where load rating becomes critical is for the vehicle OEM during the design phase. In the U.S., all cars must meet FMVSS 110 which places requirements on the minimum load rating of the tires a car is equipped with. In a nutshell, for passenger cars, there are two things to worry about. First, the maximum load on the front or rear axle must be no more than the combined load ratings of the two tires on that axle. Second, the “Normal” load shall be no more than 94 perfect of the combined load ratings of the two tires on that axle. Normal load is defined depending on the maximum number of passengers the vehicle is designed to accommodate according to this chart:
If a passenger tire is used on an MPV, light truck, bus, or trailer, then the load rating is first divided by 1.10 before the above rules are applied.
The last part of the load rating shown on the tire sidewall is a refinement of the speed rating. For my Mustang, the speed rating in the tire size is “Z” which means the tire is certified for over 149 mph. The “Y” shown in the load rating refines this, and means the tire has been certified and tested up to 186 mph. Per truck body manufacturer Reading Truck “Y: The fastest speed rating — once Z which indicates faster than 149 mph — now is Y, which allows the tires to reach 186 mph.”
You may find on your tires that there is no speed rating in the tire size, in which case it will only be shown as part of the load rating.
The next marking on my Mustang tire says “EXTRA LOAD.” There are essentially two load ranges in passenger car tires: Standard Load and Extra Load. Standard Load tires have a load rating based on 35 psi inflation pressure while Extra Load tires have a load rating based on 41 psi and contain reinforcements in their construction to handle the higher pressures and loads. Usually this means the load rating for a XL tire is 3-4 load rating points higher than a standard load tire of the same size. For my Mustang tire, for instance, the XL tire has a load rating of 99 while the same size in a Standard Load would have a load rating of 95. That’s a difference in maximum load of 85 Kg. That’s probably not a big deal in the case of my Mustang but it could be a big deal for a different vehicle.
Max Load and Pressure
Next is the maximum load the tire is certified to as well as the maximum pressure the tire is designed for. Note that this is the maximum pressure the tire can handle and has nothing to do with the pressure the OEM recommends for that particular car. Always inflate your tires to the pressure recommended by the OEM, NOT what it says on the tire sidewall. The maximum load value is really redundant with the load rating since the load rating 99 corresponds to 775 Kg. Both of these markings are required by law in the US and Canada.
Looking further, you see the Traction, Treadwear, and Temperature ratings. These are sometimes referred to as the Uniform Tire Quality Grades, or UTQGs. These are ratings given by the tire manufacturer based on standard testing procedures to allow a way to compare tires, but are not generally very useful since they allow a fair amount of judgment on the part of the manufacturer. Also, the tests are not always realistic or representative of modern driving conditions.
For instance, the treadwear rating is a measure of tire wear compared to a reference tire. Tires are run on vehicles in a convoy through a 400 mile test loop in Texas. The wear of the test tire is compared to a standard tire run under the same conditions. If the test shows that the test tire will last the same amount of time as the reference tire then it gets a rating of 100. If it is expected to last twice as long as the reference tire then it gets a rating of 200, etc. The thing is that manufacturers may not over report the results but they are allowed to under report them. In other words, if the test results show a rating of 500, the manufacturer is allowed to report 400, but it may not report 600. It may choose to do this for marketing reasons, but it means the numbers you see when buying a tire are not necessarily accurate.
Similarly, the Traction grade numbers are based on a test where the tire is mounted on a trailer and the wheels are locked up on wet pavement to measure the friction between the tire and the road. The problem is that friction of a sliding tire is always lower than the friction of a rotating tire and since almost all cars have ABS these days, a tire is almost never really sliding. The message here is that you could use the UTQG numbers to compare tires but take them with a grain of salt.
Now we get to the more esoteric stuff. The next marking on my tires is DOT F30F OBDX 1319. What the presence of this code means is that the tire meets all the requirements of FMVSS 109 and/or FMVSS 139 and all tires sold in the U.S. MUST have a marking similar to this or they are not legal to be sold. In general, the DOT code is the letters “DOT” followed by 8 to 13 letters and numbers and is called the Tire Identification Number or TIN. The portions of this number that are required by law are the letters “DOT,” the plant code, and four digits which show the week and year the tire was manufactured. In our example, these digits are 1319 which means the tire was manufactured in the 13th week of 2019. [Editor’s Note: Generally tire makers recommend that their products be replaced every six to 10 years. -DT]
Next to the DOT mark are two to three digits that show the plant the tire was made in. In this case “F3” means the tire was made in the Manufacture Francaise Des Pneumatique in Roanne, France. The next two digits (which are not required by law), “0F” refer to the tire size. The remaining letters, “0BDX,” before the date code are optional and, per Cooper Tire, “show the tire type and manufacturer’s code.”
Ply Material Call-out
Also required to be shown on the tire sidewall are the generic names and number of plies used in the construction of the tire. In the case of the Mustang tire, it shows 1 Polyester, 2 Steel and 1 Polyamide ply in the tread area and 1 Polyester ply in the sidewall area.
China and Brazil Certification
Depending on where your tires were meant to be sold, you may also see a symbol that shows CCC inside a circle followed by a number. This is the “China Standard Mark” and means the tire is certified for sale in China. Near this symbol you may also see the Inmetro mark:
This is the certification of the National Institute of Metrology, Standardization and Industrial for Brazil Market and means the tire is certified for sale in Brazil.
Sometimes, as in the case of the Mustang tires, tires that are required to be mounted a certain way on the rim will have the word “Outside” printed on the sidewall which is meant to face outward on the vehicle.
For tires that are tubeless, which is pretty much every passenger car tire these days except for vintage car tires, the word “Tubeless” must be included. For tires that require innertubes, it will say “Tube Type”.
If you’re curious how an inner tube tire works, check it out:
Modern tires actually create an air-tight seal between the tire’s “bead” and the wheel, with pressurized air filling the space between the rubber and aluminum (or steel). Here’s a look at the beed:
Discount tire describes the bead, writing:
The beads of a tire form the contact point between tire and wheel. They are made with high tensile strength steel wires and are surrounded by hardened rubber compound. They ensure an airtight seal between the wheel and the tire. Bead chaffers rest between the bead and the body ply of the tires, preventing the bead wires from damaging the tire casing and improving the tire’s handling by making the sidewall above more responsive.
You can see the tire bead in the images above.
If the tire is a radial tire, it must say “Radial” on the sidewall. The term “Radial” refers to the way in which the various belts inside the tire are oriented. Inside the rubber of a tire are a number of belts made from steel, nylon, Kevlar, or other materials. These belts form the structure that gives the tire its shape and resists the pressure from the air inside. As a tire rolls down the road, it has to flatten slightly where it contacts the road. This flattening means the tire has to change its shape momentarily from round to flat and the belts need to allow this to happen. Before Michelin invented the radial tire in the early 70’s, tires were made with the belts oriented at an angle or “bias.” Here is a graphic showing that angle:
Image via Kenda Americana Tire
With the belts at an angle, the movement that allows the tire to flatten against the road causes the belts to squirm against each other which causes heat to build up in the tire. Heat is the enemy of longevity in rubber and will cause the tire to wear out faster. Heat is also energy and there is only one place the energy in your vehicle comes from and that is the engine. Having heat buildup in your tires means energy is being wasted which translates directly into worse fuel economy. With the invention of the radial tire, the belt orientation was changed to run at 90 degrees across the tire from bead to bead. This means the belts can more easily accommodate the shape change as the tire is rolling down the road and there is significantly less heat build up. The result is better ride, lower fuel consumption and longer tire life.
A warning regarding the dangers of over-inflating your tires and the need to have them installed by trained personnel is often included. This warning is not required by either the US or European standards, but most tires will include some version of it. On the Mustang tire it states:
Warning: Serious Injury May Results From:
Tire failure due to underinflation/overloading – Follow owner’s manual or tire placard in vehicle.
Explosion of tire/rim assembly due to improper mounting. Never inflate beyond 40 psi to seat beads – Only specially trained persons should mount tires.
Mixing bias tires with radials on the same vehicle. Mixing different tire sizes on the same axle.
Lastly, on the Mustang tire there is an E code which is a European requirement showing the ECE certification. All tires sold in Europe must have a marking like this on the sidewall and it shows the country where the tire was certified along with a number that identifies the type approval document issued for that particular tire/size. In this case E2 means the tire was certified in France.
In addition to the marking shown on my Mustang tires, there are several others that you may see on your tires.
For directional tires, i.e. tires that must rotate a particular direction, you may see an arrow pointing in the direction the tire should rotate when the car is moving forward.
If you have all-season tires, you will see the mark M-S or M+S somewhere.
Snow tires will also have this symbol on their sidewall:
Continental breaks down the difference between the two markings, saying M+S is more about the tread pattern being better for snow and mud than a typical tire, while the “alpine symbol” also called the “Three-Peak Mountain Snowflake Sympol (3PMSF)” denotes that the tire meets certain test requirements:
The original definition of M+S tires is based on the geometry of the tread design. The M+S designation was first used to differentiate the knobby, bias ply tires intended for use on muddy and/or snow-covered roads from the straight rib tires used on early cars or trucks. Tires with tread designs that meet the definition may be branded with the letters “M” and “S” in several different ways (e.g., M&S, M+S, M/S, MS, etc.) at the discretion of the tire manufacturer.
All winter tires that are marked with the Alpine symbol (pictured below) undergo the ASTM F 1805 tire test on medium-packed snow in standardized testing conditions to ensure their snow traction performance meets the minimum industry requirements to be considered a winter tire. Importantly, tires that are manufactured for medium-packed snow are required by law to perform this test and may display the 3PMSF symbol on the sidewall.
As you can see, there is a lot of information printed on your tires but since to most people tires are just round and black, they never really see it. So, the next time you find yourself contemplating your tires, you will now know what you are looking at. Happy staring!
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On the old Alphanumeric sizes the letter did denote both the height of the sidewall and the load capacity. It was relative to width but only for tires with the same aspect ratio
For example the good old “F”and some close choices in modern sizes.
F78 – 195 (80 series) or 205/75 depending on which side of the line you want to go.
F70 – 215/70
F60 – 235/60
So it essentially did the +0” calcs for you as you went down in aspect ratio the width grew to give you a similar overall diameter. IE you could replace the F78 with a F60 and have a very similar tire diameter and load capacity.
“Editor’s Note: Generally tire makers recommend that their products be replaced every six to 10 years. -DT”
Given that I’ve been an auto enthusiast for about 47 years and only learned how to decipher tire date codes a few years ago, I can’t for the life of me understand why the law doesn’t require tires to say “Replace by xxxx” on them. Because I assure you 90% of vehicle drivers have no idea that tires are date coded, much less how to decipher a set of four non-descript numbers on said tires.
As far as the average motorist is concerned, if there’s tread, the tire’s still good.
and believe it or not i’ve gotten into arguments with people who insist their 30ish year old tire (cant tell exactly the age because date codes weren’t standardized then….) are in perfectly good shape because the cracks are only on the surface…. The worst part is according to PA state inspection they are indeed good enough to pass.
I mean you could probably get a tyre to last decades if you kept it in an inert gas and rotated it occasionally or on a hook.
“F70-15. The letter denoted the tire width, with A being the narrowest and getting progressively wider as you move up the alphabet. The two-digit number is the load range, with higher numbers meaning higher load capacity” No, the two digit number indicated the aspect ratio, just like it is today. It may be indirectly related to the load range.
Yes, you are correct. An edit is in progress.
Some OEM tire specs confuse me. As a small example, My 2012 Prius v stock tires have a load/speed rating of 91V.
The V is for 149 mph.
It’s a Prius. A regular Prius can reportedly hit 115, and a Prius v (wagon variant) can hit 106. (Don’t, uh, ask me how I know.) So why would they choose/design it around tires of that high of a speed rating?
From everything I found (and in my lived experience since), I was okay going to a set with the exact same dimensions but a 92H rating (Hankook Kinergy PT). Really wanted all-seasons with a significant mileage warranty.
There are many reasons why a particular tire is chosen by the OEM and it may be that that particular tire had the best combination of characteristics for the Prius and just happened to have a speed rating much higher than the capability of the car. It similar to the load rating of my Mustang tires being much higher than the vehicle weight.
The Michelin TRX system used in the 70s and 80s is the rare exception to inch sized wheels since a combination of French origin and a need to distinguish them from conventional wheels meant TRX wheels are metric and both wheel and tire sizes are called out in millimeters.
I had a 79 Mustang Pace Car with these tires, of which I was unaware at the time of purchase. The car rode like a grain wagon but was a riot to drive when it smoothed out above mph.
You forgot to mention the triangles that indicate the position of the tread-wear indicator bars.
This sizing system has always driven me nuts. You can’t change your tire width or wheel diameter without also changing the tire diameter (and therefore changing the accuracy of your speedometer).
Why not have a simple tire diameter instead of aspect ratio? A 225/45R17 could instead be a 225/25R17… Then if you wanted a wider/narrower tire, you could just simply change it to a 245/25R17 or 205/25R18 or whatever.
It makes no sense to me to add the extra math here, and it would seem to make things more difficult at every point of interaction with tire size. For instance, on my MK7 Golf, 3 sizes were used from the factory, 205/55R16, 225/45R17, or 225/40R18. Their outer diameters are 24.9″, 25″, and 25.1″, respectively. Either VW independently changed the speedometer calibration for the different sizes, or just let them out into the world with these (admittedly small) variances.
Small difficulties, but totally unnecessary.
Sorry, but I’m OCD, and I’ve never vented my frustrations about this before.
VW did absolutely no changes to the calibrations. First of all, speedos are only approximate anyways. Second, differences in tire diameter of 0.2″ is less than the change from incorrectly inflated tires. Likewise, the wheel width can have the same effect. Tire diameter will also increase with speed due to centrifugal forces. So for all intents and purposes, those tires are all the same diameter.
Off-roady tires were often sold as Total height x total width x rim size. I remember buying 31 x 10.5 R15 for my old Jeep. That seemed like the best measurement. You got height, width and rim size all in the same units with no math to determine anything. I hate having to multiply by aspect ratios and convert mm to inches to tell how tall a damn tire is.
I always thought this was a more sensible sizing system, but I’ve only seen it on drag slicks, off road tires, and small trailer tires.
Considering the era of the MK7 they probably do change the calibration depending on the size of the tire on the car from the factory. Its all in the code now so it is just a matter of changing a few digits in one line of code. For example on my F-250 all the stock sizes are baked into the computer and I can use ForScan (or the OE tool) to choose between those sizes. You can also hard code for sizes that were not offered from the factory.
Or they just do the calcs for the largest diameter and let those others over report even more.
I have OBD11, and I can change it. I actually own 2 Golf MK7s, one with 16s and one with 18s, so I could check the coding. But I can tell from my speedometer vs. my speed on Google Maps that they calibrated it for the 18s. The car with the 16s is going slightly slower than the GPS.
Most cars are set up to slightly over report speed. Actual speed of 58mph when 60mph is indicated is pretty common.
Given what appears to be the readership and leadership here, it concerns me that you did not discuss date of manufacture. What is this site’s policy for a journalist’s freedom to call out their boss(es)?
Perhaps you overlooked that since those markings had worn off. /s
It’s right there in the article. I added an ed note about how manufacturers tend to recommend tire replacement between six and 10 years from that date.
Hah! I didn’t catch that. I saw DOT and my eyes just glazed over. 😉
Still, ten years, wow…
Don’t they have date of manufacturing on it?
I learned a lot from this article. There’s way more info on the sides of the tires than I realized, and I appreciate the depth of this article.
The one thing I did know is the date codes and how important they are. If you have trailers or cars that don’t get driven much, you will run out of time on those tires long before you run out of tread. I replace my camper tires every 5 years, and some people I know do it every 4 year. Same with the MG, since it only gets about 2000 miles on it a year. And I cover my camper tires when they are not going to be used for a while to help them last a little longer.
That reminds me I’m about due to replace my trailer tires again. I waited too long last time and broke a belt in one of them, which allowed the tire to expand and rub through the fender liner before I caught it.
What about the yellow and red dots. I know they are not printed, but painted. Still on the tire though.
You’re supposed to match the dots to the valve stem location on the wheel…or at least that’s what I was taught back when. I often see it not done that way though so could be wrong.
If the tire had a single red dot sticker when new then you were supposed to line up the dot with the valve stem. This helped get tire balance close when mounting.
About 5 years ago in a tire place a guy was insisting he was getting H78/14 tires or nothing, I suspect he got the latter
Since this post is about tires, I’m going to ask a generic tire question:
If you increase tire widths from stock, e.g., from a 235 to 245 section widths, could you/should you decrease tire pressure? Increasing the width means that at the same pressure, the wider tire will deform less in the direction of travel. Seems like lowering the pressure would allow you to match the size of the contact patch front-to-back while increasing the contact patch left-to-right, thereby increasing traction, which is presumably a desired outcome of going to wider tires.
The answer, like most engineering questions, is it depends. There is an interaction with the sidewall where wider tires can actually lower your load rating and require higher pressures to be safe.
You don’t, and you really shouldn’t try to get tire size to correlate with pressure. When changing width you need to change the aspect ratio as well if you want the diameter to stay the same. Say going from a 205/75/15 to a 215/70/15 would keep the overall diameter pretty close. (also, not all tires/brands are the same, there are some variances between brands in the overall shape/contact patch/tread width)
On pressure many will point to doing only what is shown on the tire placard that shows your recommended tire size and pressure. But I’d take that with a grain of salt, as it will also depend on the tires. For example I put on my 2002 LT265/75/16 vs the original 245/75/16, the recommended pressure is only about 35 PSI. But the bigger LT rated tires have a high load rating, and max at 80PSI. I usually keep around 50-60 depending on what I’m doing, but the 35 would be way too low and I’d end up with poor wear and deformed side walls that would look like I was driving on flat tires.
Yes when you change to a different size tire you need to determine the correct pressure for that tire on that vehicle. To do this you want to consult a Tire Load and Inflation Index. This one is good as it walks you through the procedure with examples starting on page 7. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjpn73z-577AhUKMzQIHWIrBhAQFnoECCQQAQ&url=https%3A%2F%2Fwww.toyotires.com%2Fmedia%2F2125%2Fapplication_of_load_inflation_tables_20170203.pdf&usg=AOvVaw0zaHumL9xB01d0CRYFLPCS
And it isn’t just changing sizes that you should consult that guide, also useful for changing between the different tire types. P-Metric, P-Metric XL, Euro, Euro Reinforced Load, LT-Metric, and Flotation sizing.
For example the old school 235/75-15 at 32psi and max pressure
P 235/75-R15 32psi – 1940lbs, 35psi – 2028lbs
P 235/75-R15XL 32psi – 1940lbs, 41psi – 2183lbs
235/75-R15 105 32psi – 1841lbs, 36psi 2039lbs
235/75-R15 109 Reinforced Load 32psi – 1819lbs, 42psi – 2271lbs
LT235/75-R15 35psi – 1530lbs, Load Range C 50psi, 1985lbs, Load Range D 65psi – 2335lbs, Load Range E 80psi, 2775lbs
Note with the Euro sizing it is all about that load index number, not width/aspect ratio.
Great article, and loved the pics/diagrams especially. Your (rare) skill at combining clear prose to complement your visuals always helps me understand things so much better!
Does anyone still make bias-ply tires at this point? Or is the required “Radial” marking an outdated part of the reg?
Coker has many bias ply tires for the purists and bias-look radial tires for the near-purists. I doubt anyone uses bias tires on new vehicles but I dunno.
You know, I wondered if Coker was actually making real ones still and not just radial look-alikes.
That is something else, that there’s still a market for them. I’d have assumed people would be willing to live with something that provides modern safety & benefits for the price of a discrete “radial” molded into it.
For the real old stuff, there’s no choice. Good luck finding a radial 33×5 for cars from the teens.
in the motorcycle world there are brand new bikes that come with bias ply tires and every major manufacturer makes them
Superb article which will have me reading my tyres.
One small point, when you say “Michelin invented the radial tire in the early 70’s” I think there’s a typo. I believe they developed them in the 1940’s, and fitted them to Citroens. The DS was a famous (mid 50s) example.
By the mid 60’s they were relatively mainstream (Rover 2000 P6s famously had Pirelli Cinturatos) and there was a long standing story (which may or may not be correct) that Mike Hawthorn, F1 World Champion in 1958, lost control of his Jaguar MkII because of mixed cross plies and radials.
Correct. They introduced the first commercial radial in 1948, and fitted it to the Citroen 2CV. Since Citroen was owned by Michelin at the time. The first American car to be offered with radials as standard equipment was the Lincoln Continental MkIII in 1970.
Appreciate all the excellent information.
I would add the following to the ECE information you already provided.
The E-marks as shown on the tire represent two different type approval markings, one for UNECE R30 (for passenger tires) or R54 (for commercial tires) and one for R117. The first two numerals “02” represent the series level of the UNECE regulation, in this case, UNECE R30.02 and R117.02. The remaining five (5) additional numbers represent the type approval number, which was granted to Michelin by the French type approval authority. The sequence of numbers and letters following the second line represent the level of compliance within UNECE R117.02 that the tire meets. S=Rolling Sound Emission, W=Wet Adhesion, and R=Rolling Resistance. The “2” after S and R indicate that the tire complies with the 2nd stage of those two particular tire performance characteristics.
Is it just me? The words to the left of the Tire Bead image read like a portion of the credits in a Binky* video…
*Bad Obsession Motorsports type: NOT a cute pacifier one from the Tea&Crumpets crowd
Truck tires often have additional load range stuff like C, D, E on top of the 2 or 3 digit code.
Don’t forget floatation sizes like:
Overall diameter, width, rim diameter.
Honestly, I’d prefer all tires were sized this way. The big downside of 355/30ZR19 or whatever is needing to do math to get the overall diameter. I’d argue that aspect ratio is much less critical to know immediately.