Hello fellow Autopians, and welcome to another edition of Ask An Engineer. This time, we are going to answer a question sent to me by Thomas who asked me why the 2023 Honda Accord Hybrid EX-L gets an estimated 51 mpg in the city while the other Hybrids (Sport, Sport-L, and Touring) only get 46 mpg. Looking at the specs for each of these vehicles, the only difference appears to be the tires, so Thomas’ question got me thinking about the influence tires have on vehicle efficiency both for ICE cars and EVs. It also reminded me of a conversation I had years ago with a friend who owned a second gen Prius. He had put summer tires and new wheels on it a few months before because he liked the look but he noticed a definite drop in fuel economy. He didn’t have any numbers to share but he said it was very noticeable. The key to understanding what’s going on here is the concept of parasitic losses.
Parasitic Losses
Whenever energy gets converted from one form to another, such as from gasoline to twisting of an engine’s crankshaft, or from electricity to the rotation of the motor’s rotor, some of the energy will always be lost in the form of heat. There is no perfect energy conversion in this world so some of it will always end up as waste. In the case of a typical ICE car, about 66% of the energy contained in each gallon of gasoline gets lost to heat and only 33% ends up creating motion for the car. EVs, on the other hand, are much more efficient, with over 90% of the electrical energy going into motion and less than 10% getting lost to heat.
Unfortunately, that’s not where the energy losses stop. Between the motor and the road, there are more mechanisms such as transmissions, bearings, seals and lubricants, and each of these has a certain amount of friction which eats away at the energy being produced by the motor. Modern bearings and seals have very low friction, but it’s not zero. Add up all the different bearings and seals in the drivetrain and they result in a pretty significant amount of friction, and as we all know from rubbing our hands together, friction produces heat. All this heat represents energy that is dissipated to the air. Of course, this energy has to come from somewhere and the only source of energy in a car is whatever is in the tank, be it gasoline, diesel, or electricity.
All these little energy losses are like little parasites living off the lifeblood of your car, like a tick living of the blood of a dog, hence the term “parasitic losses”.
One of the largest energy losses comes by way of the brakes. We can’t really call the brakes parasitic losses because we actually get some useful value out of them, but they still cause energy to be lost. When you hit the brakes, the energy of motion is converted into heat which is then dissipated into the air. This is of course the sole function of the brakes: convert motion energy (kinetic energy) into heat energy. And since the energy of motion is now reduced, the speed of the vehicle is reduced. But what if we could convert the energy of motion into some other form of energy that we could reuse later? This is what a hybrid vehicle is all about. It’s also what regen in an EV does. In both cases, the electric motor that is part of the drivetrain is turned into a generator, and instead of using brakes to turn motion into heat, they turn motion into electricity to be stored in the battery for later use. This is a fundamental concept behind how hybrids work ,and key to why they get so much better fuel economy.
Having said all that, and as interesting as all that may be, how does any of this relate to Thomas’ question? The vehicles whose fuel economy figures he’s comparing are hybrids, so that can’t be what’s going on here. As I mentioned earlier, the only difference between the vehicles in the Honda lineup appears to be the tires, and this gets to the biggest source of parasitic losses in a car: the tires.
As a tire rolls down the road, the deformation that occurs at the contact patch (temporarily transforming the round shape of the tire into a flat area against the road) and the movement of the rubber-blocks against the road surface absorb some of the energy that is moving the car forward. This energy gets converted into heat, which is the reason tires are warmer after you drive on them for a while. This concept is called the rolling resistance (RR) of the tire.
Tire Rolling Resistance
All tires have some RR, but the amount varies widely, and is a function of (among other things) the chemistry of the rubber used in the tread, the construction of the belts and cords, and the tire size. Summer tires tend to have higher RR due mainly to the chemistry of the rubber that gives them such high grip. All-season tires tend to have lower RR, but then they also have lower grip. Wider tires tend to have higher RR while a larger outer-diameter (not wheel diameter) tends to reduce it. This is why you see large narrow tires on cars such as the BMW i3.
Since tire manufacturers started putting silica in tire rubber, RR has been markedly reduced without compromising other things tires need to do, like provide grip in dry and wet conditions, and absorb impacts from the road. Modern tires use the latest technologies in construction and rubber chemistry to reduce their energy consumption while still maintaining good grip. In the past, low RR was inevitably accompanied by poor traction, especially on wet roads. These newer tires still have some similar compromises, but they are getting much better, and I’m sure we will see many more developments in the future that make them even better.
Let’s look at RR in more detail. This diagram shows a tire with a flat portion on the bottom where it meets the road. This is the contact patch. It’s quite exaggerated here but that’s just to illustrate the point.
Imagine a piece of the tread as the tire is rolling down the road. At first, the piece of tread is round, but as it comes around and meets the road, this piece of tread has to change shape from round to flat. Any time we ask a material to change shape, it takes energy. The same happens here’ we have to put energy into the tire to make the tread change shape. When our piece of tread reaches the end of the contact patch and returns to a round shape, we will get some of that energy back, but not all. The bit of energy that is lost turns into heat and warms up the tire.
The other thing that is going on is when the tread is asked to change from round to flat, is that materials of different lengths are now being asked to be the same length. Let’s look at the angle between the start and end of the contact patch. It’s about 60 degrees:
Let’s look at a 60 degree section of an undeformed portion of the tire. Notice also that the belts (shown in yellow here) and the tread are not the same distance from the center of the tire. If we assume our tire is a P235/50R18, then the distances might be something like this:
The length of belt (Lb) and the length of tread rubber (Lt) contained in our 60 degree portion of the tire are not the same length, since the tread is farther outboard. In this case, the length of belt is about 13.61″ while the length of rubber is about 14.27″. That’s a difference of over 5/8″. Now, when this same 60-degree portion of the tire rolls around to become the contact patch, both the 13.61″ length of belt and the 14’27” length of rubber need to fit into the same straight length of contact patch. Just looking at the bottom of the picture above, you see that the tread and belt are parallel, and thus the same length since they’re no longer arcs at different distances from a center point — they’re lines.
And since the belts are made of steel and Kevlar and other strong materials, they do not want to change significantly in length, so the rubber becomes forced to conform to the smaller 13.61″ length of the belts and gets squeezed in the process. This squeezing doesn’t happen all at once, but slowly progresses as the tire rolls over the contact patch. The result is a lot of squirming of the rubber between the belts and the road surface, causing more heat to build up in the rubber. If there is a lot of space between the tread blocks, like in our diagram or like there would be in an all-season tire, the rubber has somewhere to go as it is getting squeezed, but if the tread blocks are close together (i.e. so there’s more material touching the ground) like in a summer tire, the rubber has less room to move around. This is part of the reason summer tires tend to have higher RR than all-season or winter tires.
RR is designated by a unitless number that represents the force required to roll the tire forward for every 1000 units of force the tire is carrying. For example, if a tire is carrying 1000 lbs of weight, and it takes 10 lbs of force to roll this tire forward, then its RR is said to be 10, or 10 lbs/1000lbs. In the metric system this tire would have a RR of 10 kg/ton. Most everyday tires these days have RR numbers between about 8 and 12, with high performance summer tires at the high end of that range and all-season tires at the low end. There are tires that are lower than that, and they are becoming more and more common especially now that EV’s with their high range requirements are becoming more popular.
Of course, all of this is an over-simplification of what’s really going on inside your tires, but you can see how they would have an impact on fuel economy and EV range.
In fact, according to Seong-su Kim, a senior researcher at Hyundai Motor Group, reducing tire RR by 1.o leads to a range increase of about 5%. That’s a big deal; on a car with 250 miles of range, that’s over 12 miles. As a result, when Hyundai was developing its new E-GMP platform, to ensure adequate range, the team’s target was a rolling resistance no higher than 6.5.
While an RR of 6.5 is considered to be very low, it is certainly possible with the latest tire technology. It just depends on what you are willing to compromise to get it.
For more on the impacts of rolling resistance and tire size, Engineering Explained does a good job (see above) going into the details although he ends up concluding that wheel diameter has an impact on range based on various Tesla range specs. The reality is that it all boils down to tire RR. If we had a 19″ tire and a 21″ tire with the same RR, the range impact would be zero.
The Honda Tires
So, how does this all relate to the Honda Accord Hybrid? Since tires are some of the biggest energy hogs in your car, changing the RR of your tires can have a dramatic effect on your fuel economy or EV range.
According to Tirerack.com, the OEM tire for the Honda Accord Hybrid EX-L is either a Hankook Kinergy GT or a Michelin Energy Saver A/S, both of which are P225/50R17, while the tires for the Sport, Sport-L, and Touring is either a Michelin Primacy MXM4 all-season or a Goodyear Eagle Touring all-season in a P235/40R19 size. The Primacy MXM4 is the same tire we used at Tesla on the Model S and at that time, if my memory serves, the RR was about 8.5.
Let’s suppose the rolling resistance of the EX-L tires had the same target as Hyundai used for their E-GMP platform. This mean a RR of 6.5, which is 2.0 lower than the Primacy MXM4 tires on the other cars. Based on Hyundai’s numbers, this would translate into about a 10% improvement in fuel economy, which is exactly the difference between the EX-L and the other Honda hybrids EPA ratings.
For a modern ultra low rolling resistance tire, 6.5 is not an unrealistic number, so it is entirely possible that the improvement in the fuel economy of the Honda Accord Hybrid EX-L is solely due to the difference in tires the car is equipped with. It really does make that much of a difference. Of course, to get such a low RR, it is possible that Honda had to make some compromises in performance, which they felt were unacceptable on the higher end hybrids. This may explain why they didn’t put the same tires on all variants.
The difference in range due to tires is evident at Tesla as well. The base Model S with 19″ tires has an EPA range of 405 miles while the range with the larger 21″ wheels is 375 miles. The difference is that the 19″ tires are all-season low RR tires while the 21″ers are high performance summer tires with more rolling resistance.
Another example of the impact tires have on fuel economy and EV range is the difference between the range of the Hyundai Ioniq 6 SE and SEL. The RWD SE comes with a P225/55R18 and has a 361 mile EPA range while the RWD SEL comes with P245/40R20 tires and has an EPA range of 305 miles. According to Hyundai’s own website, everything else that would impact range appears to be the same for both versions: weight is the same (4,222-4,376 lbs), battery size is the same (77.4 kWh), and both use the same 168 kW/350 Nm motors. The only difference appears to be the tires, so here again we see how just changing tires can have a significant impact on EV range, just like it does on ICE fuel economy.
If you have any other car engineering questions, please send them to AskAnEngineer@TheAutopian.com and I will do my best to answer them. As my college professors used to say: “There is no such thing as a dumb question, only dumb answers”.
“Of course, all of this is an over-simplification of what’s really going on “
Understatement of the week.
The Energy Savers on my 2017 Volt (Volt-specific tread) are absolutely terrible in anything that isn’t perfectly dry weather – if someone sneezes on the road a county over, they’re going to slide in a turn…but dammit, they’re light and efficient as hell so…I guess that’s what I’m sticking with…
Actually, it’ll be the Bolt-specific Energy Saver when I get tires, which has a slightly narrower contact patch and I believe is a pound lighter…so it should be even more efficient.
Thankfully, I run Xi3s in winter, so the Energy Saver’s horrific snow performance is of no concern.
I would expect that the primary difference is simply that the EX-L has 17″ wheels while the other models have 19″ wheels. Between the extra weight of the physically larger 19″ wheels and the fact that this extra weight is primarily located at the rim of the wheel, even with the same tire diameter, tread pattern, rubber compound, and construction methods, the 17″ wheel will require meaningfully less energy than the 19″ wheel to overcome rotational inertia during acceleration.
This reduced inertia is especially important in city driving, which is where we see the biggest gap in mpg (51 city for the 17″ wheeled EX-L and 46 city for the 19″ wheeled Sport). The EX-L, with its smaller wheels has a 5mpg advantage in city driving, but only a 3mpg advantage on the freeway. If the tires’ rolling resistance were the primary driver of fuel efficiency here, the mpg advantage would be greatest at highway speeds where inertia plays very little role and rolling resistance dominates. Instead, we see the reverse. In city driving, where speeds are low and where the car is constantly starting and stopping, the effect of rolling resistance diminishes (due to the slower rotational speed of the wheels, the rubber has more time to “squeeze,” which requires less power since the amount of work to “squeeze” remains the same, but is done more slowly and the same work over a longer period of time requires less power), the effect of the wheels’ inertia increases (they need to be spun up and spun down many times due to stopping and starting), and the mpg gap between the EX-L and the Sport nearly doubles. This is best explained by the additional power required to overcome the greater rotational inertia of the larger wheels as the car repeatedly stops and starts in city driving.
That said, I tend to agree with the author that rolling resistance plays a larger role in total *range* than wheel diameter does. When most people think of range, they’re thinking of a scenario where the car is not in stop and go traffic for significant portions of the driving time. This means range is generally considered under largely steady-state cruise conditions where the wheels’ inertia is overcome once when the car gets up to speed and then becomes (mostly) irrelevant. In that scenario, rolling resistance is king.
You bring up some good points, but it’s worth noting that at higher speeds, aero becomes (exponentially) the largest loss. You’d expect the gap to close since both cars should have similar aerodynamic drag, and proportionally the rolling resistance difference (or whatever other factors are at play) become a smaller portion of the total.
Granted, the 19″ wheels probably also have worse aerodynamic drag.
Tire diameter also has a huge impact – I put lower profile tires (70 profile…) on my little kei car, and it got ever so perky. I think my range shrunk considerably, but since the odometer doesn’t know how big the tires are, it gave the wrong readings too.
This is really great, thank you for the close look at how tires work.
My car (the opposite of an EV, 1960 Rambler) came with 6.00×15’s, comically skinny. The narrowest/tallest tires I could find are Toyo 215/75-r15’s, I need the ~ 28″ diameter for gearing reasons.
It’s impossible to find usable (daily-driverable) skinny tires or tall sidewall/profile tires. The Japanese Toyos are “small truck” tires, and very heavy. I run them at 40 lbs.
The EV product emphasis on performance and no frugal small car/long range/low performance leaves me utterly uninterested in EVs (never mind cost).
My sports car, OK, there I stuff as much rubber as possible; different goals. But for dailies, I’d love much narrower tires.
Thanks again for the close technical discussion.
I would also suggest that tread and weight have significant effects on both RR and power loss. In the off road community, the RR is greatly effected by the agressive tread patterns, and power loss that would equate to gas mileage can very greatly with tire weight. It’s not uncommon in truck tires for the weight to be one of the biggest factors in gas mileage- even the same tire can have significantly different mileage performance based on it’s age. My 35 inch mud tires are not only lighter after 30k miles, the tread is less aggressive and the circumference is less- all effecting gas mileage and performance.
EXCELLENT article!
I put Michelin CrossClimate2s on my wife’s Fusion Energi and have seen some reduction in MPG from the Bridgestone Ecopia 422s. The all-season grip has been worth the tradeoff, but maybe not for everyone. Tire Rack did a great head-to-head test with a Tesla: https://www.tirerack.com/tires/tests/testDisplay.jsp?ttid=298
Wondering where weight is in this article. Maybe I missed it as I skimmed through. Definitely has an impact.
The flip side to a decrease in range might also be more important, the impact the particular tire has on handling in the dry, wet, snow etc you deal with on a daily basis. If I have to lose a little range to get better traction and grip, I will gladly do so for the purpose of fun and primarily safety.
I wonder if future Ioniq6 and other BEV buyers can purchase a higher trim level and downgrade to the smaller wheels? I can see that becoming a desirable option which would allow them to enjoy the other benefits of the higher trim level.
This should be a factory option.
This assumes that the companies assume that buyers are educated and logical. That’s a lot of assumptions…
Also for decades companies have purposely put the small and ugly wheels on the lower/entry trims to drive buyers to the higher margin trims.
I thought the same! That needs to be an option
I owned a 1st gen Honda Insight for 13 years and learned very well the disproportionate impact of using the best low rolling resistance tires. If you wanted to achieve 70mpg the only choice was the OEM tires spec’ed by Honda: 165/65R14 Bridgestone RE92. No other tire came close. Deviating from what the engineering elves and Smurfs specified always resulted in at least a 10% loss.
Interesting.My car uses the same size tires.It would be an interesting experiment to see if that translates into similar savings on mine
Makes sense. The Insight loses less than most to idling or aero, so that leaves rolling resistance as a disproportionately high contributor.
One of the things that I’m most interested in is how A) wheel design and B) wheel weight affect efficiency. I have a Subaru Crosstrek that I’m considering adding aftermarket wheels to. They would be the factory diameter and width, but of course would have a different design and weight.
Lighter wheel will always be better. Less unsprung mass means better ride, although road noise might get worse since the tire is absorbing less of the road energy. Lighter wheels are easier to spin up so will take less energy to accelerate, especially in a non-hybrid car like yours. Keep the diameter and width the same as stock and you should be OK.
One thing not addressed here is how weight affects rolling resistance. It’s well understood by bicyclists that if you want to reduce the weight of your bicycle, first start with the wheels and tires. Because the weight of the spinning wheels saps about five times more energy than the weight of the rest of the bike. Of course, this mostly affects acceleration and inclines, since weight has very little effect on simply maintaining speed on flat ground.
Weight definitely has an effect on rolling resistance since the actual force it takes to roll a tire is directly dependent on the load it is carrying. Less load means less force to roll the tire.
I’d like to point out that an extra 2lbs of weight in the tire pales in comparison to the ~750-1000lbs each tire is carrying. As to whether the magnitudes of forces at play are comparable between cars and bicycles, I can’t say.
It’s not the same thing. Easiest way to demonstrate is that you put small weights to your feet and take a sprint. It’s quite different than having those same weights in your back.
I’ve always said that everything I know about driving a car efficiently I learned on a bicycle.