Smaller wheels highway question?

[Phase]

https://youtu.be/NYvKxsYFqO8?si=8xPyLFsIlT_hvxvh

A decent break down of wheel sizes and how it affects range, especially in city driving

Then he also states steady state at 75 mph and says that tires are more aerodynamic than rims with spokes. At first you’d think okay a flat disc wheel cover fixes the problem but then you also have the inside of the rim facing the wheel well also spinning and pumping air around too

[Ecky]

Difficult to isolate the various factors. What we do know is, larger diameter wheels (with the same circumference tire) have the following against them:

1. Heavier
2. Larger spoke/barrel exposed to wind
3. Often come mounted with different (much higher RR) tires
4. Come on different trims of cars, which may be heavier or have worse aero

Ultimately, you could do some testing to try to isolate each of these and tell us what factor truly is the largest. You could do any number of the following:

-Drive two different trim vehicles (Prius with 17" vs 19" for example) which come with different factory tire sizes. The EPA "highway" test has a lot of low speed driving, accelerating and decelerating, and may not be representative of true open road performance.

-Put the 17" wheel/tires onto the vehicle trim that came with 19", and see if the higher trim matches the economy of the lower trim.

-Swap equivalent GT tires onto the 17" wheels, or equivalent 19" eco tires onto the 19" wheels, and compare with the stock tire, and see if the LRR tires make a difference on the highway.

-Compare 17" wheels with 19" wheels when using the same tire series (e.g. eco tires on both).

-Add weight to the 17" wheels to make them weigh the same (or have the same moment of inertia) as the 19" wheels.

-Add some sort of plastic ring to 19" wheels to simulate the smaller aero profile of the 17's.

~

My money is on the tire RR being the biggest factor, even at highway speeds, followed by aero, followed by weight - most modern cars have air curtains over the front wheels anyway. I'd like to be surprised, however.

Regardless, we can say for certain that a smaller, lighter wheel with lower rolling resistance tire, has in sum a measurable positive impact on fuel economy.

[Piotrsko]

Only thing I would change in the above is the larger rim had more mass, not necessarily heavier unless proven wrong by a scale.

However really nice summation of removing factors. Didn't address areo result from ride height changes

[freebeard]

17:39 Thixomolded Magnesium // Wheels

That was then, this will be tomorrow.

[Phase]

https://www.caranddriver.com/feature...-wheels-tires/

They did a test and showed better gas mileage with smaller wheels. Kept tires identical accords the board

[Ecky]

Originally Posted by Piotrsko

Only thing I would change in the above is the larger rim had more mass, not necessarily heavier unless proven wrong by a scale.

However really nice summation of removing factors. Didn't address areo result from ride height changes

Shouldn't be any ride height change, given they're larger diameter "rims" fitting a tire with the same outer diameter. You get a higher moment of inertia but the overall rolling circumference is the same.

[Ecky]

Originally Posted by Phase

https://www.caranddriver.com/feature...-wheels-tires/

They did a test and showed better gas mileage with smaller wheels. Kept tires identical accords the board

That at least isolates the vehicle trims, EPA test methodology, and tire properties. We still have weight and aero as the remaining factors.

[aerohead]

Originally Posted by Phase

People claim that bigger and heavier wheels help with highway gas mileage because of inertia

If that’s true, then why are all of the epa highway numbers better with trims that have smaller wheels? Wouldn’t just the city mpg be better and the highway mpg be worse?

So are bigger wheels better or worse for highway mpg?

I've only seen the argument that, the 'smaller' wheel/tire combo is better aerodynamically, with lower 'windage'/ ventilation drag.
EXAMPLE:
Hyundai IONIQ 6 SEL & Limited, with 20" wheels = Cd 0.246
Hyundai IONIQ 6 SE, with 18" wheels = Cd 0. 219
A 12.3% difference in Cd.
--------------------------------------------------------------------------------------
One of our members is presently experiencing range loss on his Genesis GV60, because of the 'larger' wheel/tire combo.
--------------------------------------------------------------------------------------
The 'slightly-convex', 'closed' wheels on GM's 1987 Sunraycer demonstrated zero ventilation drag at all, at crosswind ( yaw ) angles below 15-degrees.

[Phase]

Originally Posted by aerohead

I've only seen the argument that, the 'smaller' wheel/tire combo is better aerodynamically, with lower 'windage'/ ventilation drag.
EXAMPLE:
Hyundai IONIQ 6 SEL & Limited, with 20" wheels = Cd 0.246
Hyundai IONIQ 6 SE, with 18" wheels = Cd 0. 219
A 12.3% difference in Cd.
--------------------------------------------------------------------------------------
One of our members is presently experiencing range loss on his Genesis GV60, because of the 'larger' wheel/tire combo.
--------------------------------------------------------------------------------------
The 'slightly-convex', 'closed' wheels on GM's 1987 Sunraycer demonstrated zero ventilation drag at all, at crosswind ( yaw ) angles below 15-degrees.

Well that’s nice to know it helps drag too

I plan on getting the awd Prius and it takes a 2 mpg hit for both city and highway versus the front wheel version

Switching from the stock 17s to some after market lighter 16 inch wheels should help get back some of the city driving mpg and I guess it’ll help with highway too. Not to mention I’d be doing fully enclosed custom aero caps on the 16s

[aerohead]

Originally Posted by Phase

Well that’s nice to know it helps drag too

I plan on getting the awd Prius and it takes a 2 mpg hit for both city and highway versus the front wheel version

Switching from the stock 17s to some after market lighter 16 inch wheels should help get back some of the city driving mpg and I guess it’ll help with highway too. Not to mention I’d be doing fully enclosed custom aero caps on the 16s

Unless the 16" tires have lower R-R, the smaller wheel diameter wouldn't necessarily have an aero advantage, and the difference in mass would hardly register on the radar, compared with the overall mass of the car.
On full BEVs, the single motor, large pack variant is typically chosen for highest range.
In town, the larger mass of the 17s would offset some acceleration penalty with more regen kinetic energy harvest.
Only the mathematics would tell.