It's true that tire diameter and weight will affect the fuel mileage, but not really enough to matter, unless you go really small (which will look funny on your 4wd).
I went DOWN from 245/75/16 stock alloys BFG Long Trails and put on 235/75/15 Dueler AT Revos on Explorer alloys, and gained a few mpg, but I LOST them again when I put on Michelin LTX MS, same 15" size. Just a heavier tire made an mpg difference of 20-30 miles per tank (which is all I ever look at).
What you WILL notice, should you put smaller diameter tires on, is speedometer and odometer error (only correctable by having Ford re-flash your ABS module for a 2000) and a noticeable increase in pep. It's like going to a 4.33 gear from 4.11 in my case. You REALLY notice that from stoplights and getting up to speed. When you calculate the mpg, you'll have to correct for the "actual" mileage. Otherwise it'll LOOK like you gained a LOT of mpg because the odo will be reporting 5.5% more miles that actual, using my example.
I don't think you will be very happy if you are buying smaller wheels and tires JUST to gain mpg. I think whatever mpg you do gain (2, MAYBE 3 mpg) will get eaten up from happy right foot with the gear ratio "effective change".
Gee, my top speed limiter lets me go to 96 mph now.....not so fast there, math whiz.....it's still really 92.
You'll be money ahead just coasting up to red lights, anticipating traffic rather than jack-rabbiting and hard braking.
Last thing, I get HORRIBLE gas mileage when I put my old steel wheels with 31x9.5/15 mud tires on. The whole truck feels heavier, the ride suffers (very jouncy) and the noise is head-ache droning at hiway speeds. Those tires are HEAVY, but almost exactly the same diameter as the 16" BFG's.
So, maybe if you find a passenger tire that weighs a lot less than the BFG's, you'd see a noticeable improvement, but I can only guess how much. You really don't want much less than a 235/75/15 on a 4wd because of the weight of the truck and load-rating of the tire.
Last edited by Earl43P; 01-29-2008 at 02:14 PM.