I can attest to this. I lost nearly 4 mpg when I went from my OEM Wrangler SR-A's to BFG All Terrains KO's.

 

Can't make horsepower if you ain't burning fuel!!! Just my $.02

 

I don't think I'd be doing a whole lot of changes for 1-1.5mpg gains. In the grand scheme of things how much are you really saving?

You'd save a whole lot more by just going easy on the skinny pedal.

 

I can trade straight across for stock 18's. If i knew i'd get 1 mpg gain, i'd trade in a heartbeat. I'd save $5-$6 a fill-up, and over the life of the truck, that's a nice chunk of savings. I can't get any easier on the pedal. I drive like a grandpa trying to milk mileage out of this thing.

Nobody seems to know what the gain would be. The circumference of the wheel will be the same, but each wheel would be about 10 pounds per wheel lighter. From what i've seen the 18's are approximately 5 pounds lighter and the tire about another 5 or so.

I would believe w/out a doubt i'd see gains if i went w/ a narrower tire, but since the tire width and circuference is the same, i'm only shaving weight, and it's not worth trading rims for tenths of a mile per gallon.