Does the size of your tires effect your fuel economy? Larger tires have a greater diameter, and so for every revolution your car will be carried farther along the road. However, the larger diameter also means the torque provided by the engine will result in a lower propulsive force. The torque is the product of the tire radius and the force, so with a fixed torque provided by the engine, larger tires lead to smaller forces. With sufficiently large cartoon sized tires, the engine would not be able to even move the vehicle. So large tires may allow the engine to operate at lower RPMs during steady highway cruise, thus saving on gas. But at the same time, whenever you are accelerating the larger sized tires will force the engine to provide more torque then normal, thus forcing it to run at higher RPMs and burn more gas. Thus larger tires might help your fuel economy if you do a lot of highway driving on flat roads at constant speed. But if you are accelerating a lot, for example in city stop and go traffic or travelling on hilly highways you will probably use more gas.