Myth busted
 

I am looking for ways to increase mileage. I heard smaller tires help do this. So i put a set of 235's street treads on the truck, inflated them to thier maxium pressure of 80 psi. I have been running my mudgrip 285's. 235's @80psi= 16.77 mpg. 285's @50psi= 15.9. Not exactly what I thought I would get. Just wanted to share that with you guys. The truck looks better with the aluminum rims and 285 than the stock steel rims with the 235's. The steering is is much tighter and more precise with smaller rubber. Easier to hold a straight line The rider is harder on bumps though. FYI, truck is stock as they come, minus the mossy oak air freshner.

Uh oh, that qualifies as a heavily moddified vehicle.

You didn't compensate your distance figures for tire size. Putting on a different size tire will effectively change your gear ratio which will give you a slightly different odometer reading, which in turn will impact your MPG numbers.

Yup, really need to use a GPS to track mileage.

Ford probably would have/will void your warranty due to that air freshner. An SCT will take care of it thou! :)

When you made the change did you go to a higher aspect ratio tire (ie 235/85) to keep the diameter the same? Otherwise if the diameters were not the same the effective gear ratio of the truck would have changed effecting mileage. A narrower tire has less rolling resistance and will get better mileage. Different tire brands and tire construction will effect the rolling resitance too. There are so many other variables that an average over a period of time would have to be taken or a test run under the same conditions...road, winds, temperature, etc. One of the freight companies I used to drive for run narrow Michellins on some of the pick ups and they averaged better mileage than the other trucks. Worked better in the snow as well. JMHO