Nice Forum!!

My first post and already causing trouble. <img src="/forums/images/graemlins/nana.gif" alt="" />

In most cases, higher rpm means more air/fuel going through the engine.... less mileage. The primary reason for overdrive is to lower rpm for better mileage.

ANd it's the width and more agressive tread design that kills mileage, not the diameter of the tire. Also the gearing because of larger tires on same axles may be wrong for engine, but not always... and you need to re-gear speedometer/odometer for the change or it will show less miles then you are actually driving.

Use bicycles for an example. Fat knobby tires are harder to pedal then narrow smooth ones.

I'm running 32s on 3.73s and get good mileage (20-21mpg on highway tread, but 19-20mpg on MTs), but probably wouldn't work on 3.55s.

Good Luck! <img src="/forums/images/graemlins/cheers.gif" alt="" />