eli5: Why does changing tires to a larger size make my truck gas mileage go down?
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If you mean fuel consumption:
Look at your vehicle when static, you can see the bottom of the tire copies the shape of the tarmac, it’s not a circle, the bottom part is flattened. Wider or bigger diameter tire will increase the amount of rubber that bends to make the contact surface. When you drive the rubber of the tire so continuously bending in contact with the tarmac. When you bend an object, its resistance to bend will generate heat; its called histeresis. More bending more heat more power loss. You can counter it by inflating the tires more but there’s a safety limit to how much you can inflate tires.
Another completely different factor is extra friction from wide tires during turns. The outboard side of each tire and the inboard side of each tire are doing the turn, but on different turning cirlcles, one is closer to the turn center than the other. So the tire has to partially slip to compensate for the different turning radius performed by different parts of the same tire. Given most roads include turns, this factor will also add up to lower your mileage.
Shoulder height plays a role too. Having thin tires on big wheels may increase or decrease mileage according to the shoulder behavior. Thin tires tend to have a stiffer shoulder which will bend relatively more, with more resistance to bending, meaning a lot of heat. On the other side, it improves the vehicle response by a lot. The opposite, soft high shoulder will give more “floaty” feeling to the vehicle. It’s ok for comfort and economy, bad for sport use.
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