[Big Dave]

I have talked to hundreds of pickup guys and the trend is very clear:

Bigger diameter wheels/tire = worse MPG.

The reason is straightforward. Unless you run very long legs at the same road speed, you have to accelerate. Not only do you have to accelerate the vehicle, you must first accelerate the wheels/tires before they will accelerate the vehicle.

The polar moment of inertia goes up with the square of the diameter, so if you go from 13 inch wheels to 16 inch wheels the polar moment of inertia increases by 33%. The wheel/tire assemblies act like flywheels.

Look at the high-MPG cars available. Small diameter tires. 13 or 14 inch. No 18 inch wheels on the vehicles with high MPG.

Stay with the factory-sized rubber. They do know what they are doing.