I have 99 TJ. When I got it I would get 18 MPG with the air running, sitting on 235 tires. I put 31's on and i still got 17 or so. After that it just kept going down the tube. I know my speedo is off by 9%, but i figure that in. Now am lucky to get 15, and i drive like a grandpa shifting at 1700 to 1900 rpms, but nothing seems to help. Any sugestions?