Ring and pinion issue

I have an issue with my truck, it's a 1991 4x4 v6 Automatic. I have BFG 31's and 4.56 gears, the problem is, I am only getting 12 miles per gallon, I installed the K&N FIPK but there wasn't really an improvement on mileage. Also I try to drive as smooth as possible, with out accelerating too much. I think the truck came with 31's form factory, because the spare is a 31 and I think it has never been used. I wonder if switching to 4.88 gears would get better mileage. Any opinions??? Any other suggestions on gaining mileage, Please HELP ME. Thanks in advance.

the jump from 4.56s to 4.88s isnt even much of a difference for the price. thats about right for the motor and tranny combo. for the money u can prolly get a manual tranny and swap it in for better milage

DRew