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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
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