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Discussion Starter #1
Someone on another forum was commenting on how larger wheels don't change mileage, it's wider tires. Everything else being equal, larger wheels DO reduce mileage because it changes the weight balance, the moment of interia. Wider tires can reduce mileage, but it's less a function of the width than a function of the additional weight, tread pattern, and material used. The poster that I replied to said that he got better mileage with thinner tires. My copied reply below.

Larger wheels do decrease mileage because the moment of inertia of the wheel and tire is moved towards the outer diameter. This requires more energy to get it moving and slow it down. Tires with thinner sidewalls are more likely to have more aggressive tread materials and will further reduce mileage than tires with thicker sidewalls. Thin sidewalls are also built to be stiffer which increases weight.

Here's something else to think about - how much do wider tires decrease mileage? Since contact patch size is a function of tire pressure and the weight on that tire, assuming constant tire pressure, the size of the contact patch is unchanged, or if it is changed, is maybe only maybe 1-2% larger or smaller. The tread pattern has a greater difference in contact patch than this possible error.

Maybe the change you noticed in the mileage was from losing weight and different tire pressure (unless you live in a climate that is not seasonal). The thinner/wider tire also had a different tread pattern which changes the contact patch. The idea is that the "width" of a tire that is the main factor in effecting mileage is wrong, it's the additional weight of a wider tire, the different tread pattern and contact patch that the tire has, and air pressure.

I guess the point is that wider tires CAN reduce mileage but not for the same reasons most people think it does
 

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Discussion Starter #3
If you were to take 15" wheel+tire and compare it to a 17" wheel+tire and keep about the same rotating diameter of the wheel+tire, the 15" wheel's tire will have "taller" sidewalls but they will be thinner because they can flex more. The 17" tire has "shorter" sidewalls because it is designed to not flex as much. Regardless of the weight of the tire, the combined weight of the wheel+tire will pretty much always have a moment of inertia that is closer to the outer diameter and is unfavorable compared to the 15" wheel+tire.
 

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Maybe it's because of the contact patch size. I don't know enough to know for sure, but a wider tire has a wider, shorter contact patch compared to a narrow tire with a narrow, longer contact patch. Even if the surface area of the patch is about the same, maybe the wider patch has more resistance?
 

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That kind of makes sense...if you increase the air pressure you decrease the size of the contact patch since weight stays constant. What I don't understand is that radial tires are not like the old bias ply tires that really depend on air pressure. Some radials are even runflat, so wouldn't that make the contact patch size not a factor?
 
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I think radial tires do blow up with more air pressure, just not as much as bias ply tires, that's why you get better FE with higher air pressure and if you look at a flat tire, you can see the old tire bulge the sidewall somewhat. I bet this even applies to runflats since runflats can only run for a short while while flat.
 

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Details of Contact Patch

One source of tire rolling resistance has to do with the details of the movement of the tire outside surface, within the contact patch, as the tire rolls. Previous posts have correctly noted that the tire contact area is unchanged by the width of the tire. i.e., for a given weight on the tire & tire pressure, a wider tire will have a contact patch that is wider, but shorter and with the same area as the narrower tire. By shorter, I mean shorter along the direction of travel. This change in shape is important to remember a bit later.

As the tire rolls, the edges of the tire contact patch are pulled forward while the center tends to be pulled aft and most of the tire patch tends to scrub slightly because the diameter of the tire is slightly larger in the center then at the edges. This is one reason why a narrow tire contributes to better fuel mileage. With this understanding, one can see how higher tire pressures can contribute to better fuel mileage thru this effect: With higher tire pressure, the area of the contact patch is reduced, particularly the length of the patch, which minimizes the scrubbing.

Of course, other factors of the tire also affect fuel mileage. A narrow tire can have slightly lower aerodynamic drag. However, this is likely to be a very small effect given the airflow environment that the tire operates in. A more productive way to reduce aerodynamic drag associated with the tires / wheels would be to block the holes that allow air passage thru the wheel. But, this would adversely affect brake cooling.

Since the tire is a complex visco-elastic structure, the rolling deflection of the tire will consume energy. Higher tire pressure reduces this source of energy loss by reducing the magnitude of the tire deflection with rolling.
 

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Discussion Starter #8
One source of tire rolling resistance has to do with the details of the movement of the tire outside surface, within the contact patch, as the tire rolls. Previous posts have correctly noted that the tire contact area is unchanged by the width of the tire. i.e., for a given weight on the tire & tire pressure, a wider tire will have a contact patch that is wider, but shorter and with the same area as the narrower tire. By shorter, I mean shorter along the direction of travel. This change in shape is important to remember a bit later.

As the tire rolls, the edges of the tire contact patch are pulled forward while the center tends to be pulled aft and most of the tire patch tends to scrub slightly because the diameter of the tire is slightly larger in the center then at the edges. This is one reason why a narrow tire contributes to better fuel mileage. With this understanding, one can see how higher tire pressures can contribute to better fuel mileage thru this effect: With higher tire pressure, the area of the contact patch is reduced, particularly the length of the patch, which minimizes the scrubbing.

Of course, other factors of the tire also affect fuel mileage. A narrow tire can have slightly lower aerodynamic drag. However, this is likely to be a very small effect given the airflow environment that the tire operates in. A more productive way to reduce aerodynamic drag associated with the tires / wheels would be to block the holes that allow air passage thru the wheel. But, this would adversely affect brake cooling.

Since the tire is a complex visco-elastic structure, the rolling deflection of the tire will consume energy. Higher tire pressure reduces this source of energy loss by reducing the magnitude of the tire deflection with rolling.

What a detailed answer - someone is an engineer!
 
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