Wide tires have a greater rolling resistance, and thus reduce gas mileage.
The effect is likely negligible on a passenger car with the available tire
Increasing the diameter is the same as lowering the gear ratio. The car
will travel farther per revolution of the wheel. It can increase gas
mileage slightly, up until the point where the engine has to lug to move
the vehicle. Once again, with the tire sizes possible on a passenger
vehicle, the effect would be minor.
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