What is it about a larger tire that effects performance/mpg? Is it the additional height, width or weight?

 

Rotational mass, gear ratio, and increased vehicle height (drag).

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I'd have to say mainly the added weight. The more weight you put on a vehicle, no matter what kind of weight that is, will bring down performance. Why do you think there is so much plastic on todays vehicles?

 

^^^^^ what he said. weight, friction, drag, all that good stuff. same with driving around with just a driver vs full of passengers, gear, full gas tank, etc... the more you add, the more gas you use.

AK4Dave types faster than me.

 

weight! and the additional rotational mass doesnt help!

 

With each revolution of the tire, the vehicle travels farther than before, thus changing the EFFECTIVE gear ratio. Weight, rotational mass, and friction (due to a wider tire) also affect the performance.

 

Weight does play a factor, but so does the change in engine RPM at driving speed. Todays engines are engineered to be their most fuel efficient at a certain RPM range. When you increase your tire size, you lower the RPMs required for the same speed, and end up operating your engine outside of its designed peak fuel performance range.