So, lots of people seem to agree that wider tires are faster on bikes. The theory seems to be that wider tires run at lower pressures, allowing wheels to deform more elastically over bumps and transfer less momentum to the rider. On a bike, a wider tire also usually has a larger outside diameter, since the wheel diameter is held constant.
Cars, obviously, are very different - larger wheels are usually combined with lower profile tires to keep outside tire diameter constant. and larger wheels usually carry a significant efficiency penalty. For example, the base 2026 Prius with P195/60R17 tires gets 57 mpg, vs the higher spec models with P195/50R19 that get only 52. This video describes the same thing for some Tesla models: https://www.youtube.com/watch?v=NYvKxsYFqO8
The kia EV6 “wind” and “GT line” trims in AWD have the same powertrain and suspension design, the only notable differences are that the wind has 235/55R19 tires vs the gt line having 255/45R21 tires. And the Wind has 295 miles range vs 270 in the GT line.
There are lots of threads out there full of wrong info like “wider tires have a larger contact patch” but I can’t find anything that gives what really seems like an answer that uses actual analysis.
21" rims are usually quite a bit heavier than 19" rims and require more energy to accelerate and decelerate. Larger rims are sometimes less aerodynamic because they’re a more open design for style, but this obviously isn’t always true. The GT line would also have a wider contact patch (by about 8%) meaning higher rolling resistance (assuming the same tire).
There’s also the tire that’s run in play here. For example, on a car we recently purchased, the 22" wheel option is more energy efficient than the 21" wheel option. The reason is the stock tires on the 22" are summer tires vs. the all-season tires on the 21" wheels. Kind of like running a TT tire vs. a Conti 4 Season. (The material on the 22" wheel is also lighter in weight making them more “fragile” than the 21" wheel and less suitable for offroad, etc.)
That combination of rolling resistance, weight and aerodynamics leads to the loss of range in the car. There are a lot of factors in play w.r.t. the cars.
It’s not really an apples to apples comparison because of the influence of the change in wheel on the car, where most people aren’t changing wheels on their bikes when opting to run a tire that’s a little bit wider.
With the bike tire, you’re talking about the tire size only, so you’re ending up with maybe a higher rolling resistance, but the contact patch on a bike tire vs. a car tire is so much smaller… the advantage of running a lower tire pressure is that your tire stays in contact with the road over bumps which is always faster than running a higher pressure on a rougher surface (and also WAY more comfortable).
The interaction of the tires bulge on the same wheel is also in play in terms of aerodynamics, and certain tire sizes will be more aerodynamic than others with certain wheels. The wheels I trained on were designed for 28s but I’d run 32s on them because I didn’t care about the aerodynamics and wanted the comfort advantage in training.
There are many factors at play here, and I’m not sure which ones have the biggest impact, but to start when you’re comparing R17 vs R19 tires, you’re comparing tire diameter, not tire width. It would be like comparing a standard 700c wheel to a 32er.
You can use this calculator to find the actual tire width of a car tire:
A bigger tire will result in slightly lower RPMs at hwy speeds which would tend to increase MPG. It would also raise the vehicle higher off the ground which would negatively impact aero drag decreasing MPG. Different tread profiles also would impact fuel economy.
Also, different trim level cars may have been tuned slightly differently without that showing up in the specs (more aggressive regenerative braking, quicker transition into fuel cut when lifting off throttle, etc.)
I really think the weight thing is a red herring on the wheels - we’re talking about an 8-10% penalty in efficiency for a <1% change in GVW.
Yes, tire type definitely matters as you cited for the 22" vs 21" example you cited, but there are lots of comparisons like the one I brought up where the manufacturer is speccing the same tire in different sizes to go with different wheels - and bigger wheels are much less efficient.
Tire width is another thing often cited - but I don’t think that holds water. The video I linked in my original post calculates the aerodynamic penalty of wider tires at around 1%, and that’s really a worst case scenario, because the air under the car is already extremely dirty. Wider tires don’t have a larger contact patch - they just have a wider contact patch of the same area at the same pressure.
I think what’s at play HAS to be rolling resistance, because it shows up at steady speeds (shown in the tesla EPA data in the video I linked). But the question is - WHY do bigger wheels with lower profile tires have higher Crr?
This question really requires specific tires and/or vehicle to model and explain.
As we know with bicycle tires, width alone is not the primary determinant of rolling resistance and is only one variable, not the most powerful variable. There are very many wide tires that have significantly higher rolling resistance than narrower tires.
The relative air volume indicates the larger wheel would require significantly higher pressure to meet the required vehicle dynamics.
Eyeballing the volume is ~19L and ~14L for each respective tire wheel combo. This requires changes in how the tire is made and what it’s made of.
Lower profile tires require a stiffer, more robust casing which is balanced by a different rubber compounds of higher hysteresis to maintain ride quality, tire grip, and suspension characteristics. Because of this; the viscoelastic hysteresis would necessarily be higher.
Well that’s kinda why I said it was a combination of multiple factors with weight being just one of those (and I agree a relatively minor one at that).
You seem to have misunderstood what I said about aerodynamics. It’s not the aerodynamics of the wider tire… it’s the aerodynamics of the wheel itself. In most cases when EV shopping lately, the smallest wheels were also the most aerodynamic/“covered” wheels, whereas the larger and most stylistically appealing wheels were more open (and thus far less aerodynamic). We also see that wheel aerodynamics are a massive influence on our bikes, obviously proportionally moreso than on our cars, and usually only trailing the frame and body position of the rider in terms of importance. The difference is the speeds at which we’re generally measuring highway range/gas mileage in a vehicle are 75mph (vs. 20-30mph on a bike) and thus drag plays an ever larger role at faster speeds as it is proportional to speed squared.
So I’d disagree that it “has to be Crr” alone, but I agree that’s a big part of it as the example with my car bares out re: tire choice and we see it all the time on the road with our choices in bike tires, and even how sidewalls impact rolling resistance in the tire choices we make on our bikes. I think it’s probably primarily caused by wheel aerodynamics and Crr with a minute contribution from weight.
