I've got 4.88 gears and 35" tires. I get 12.5 most of the time on the highway. I did tow a deep hull, full windshield fishing boat to Lake Michigan and managed to still get 11.5.
My correction factor is exactly 25% more mileage more than reported on the odometer. I've checked with road markings and with GPS...
25% ?!?
With 4.88s and 35s you should be pretty darn close to stock ratios, and the speedo should be fairly close as is. I have a very hard time believing you have to add 25% to your odo to be accurate... Am I missing something? 12.5mpg sounds about right though.