ok i have a 99 xlt extended 4.0 off road w/4.10 gears, i know from the origanal sticker i have from the ford dealer and i have 245/75R16's with Yokohama Geolandar H/T-s tires and i get an average of 13-14mpg

 

Im right with you same mileage

 

wow you should downsize your exhaust i think you might be killing your truck

 

With the MODS that I have done in my signature below....along with changing everything over to synthetics; the '00 gets about 16-18 on the hwy and less (not sure) around town. Combined is usually about 14-15 mpg.
IF ALL my driving is hwy and I keep the speed down to 65-70.... I can usually get 280-300 miles on about 17 gallons.

OR 4x4

 

So should i keep the 245's or the next step higher (which i dont know)?

 

i was getting the same milage before i put the exhost on to and i like the sound its deeper:)

 

I think the problem is rolling resistance.
4.10s are a big drag when coasting
Can't unlock the front hubs,
Goofy geometry on the front CVs
31's are too short