Your speedo shouldn't be too far off, though. I have the same setup and my speedo is between 3 and 4 MPH off. Still, I get 18 mpg, (mostly city and back road driving), with 31's and the change from 30's was hardly noticeable. If you have a lift, however, I'm convined that driveshaft alignment can affect fuel mileage if U-joint angles are great. The more you throw the drive shaft off angle the harder they are to turn and I think it shows up in a reduction of mpg.