[ijames]

Do you have a handheld GPS, or a smartphone with GPS? I downloaded a free app from Samsung for my phone that shows a speedometer and odometer, so I could use it to verify that my speedometer was dead on (within .3 mph from 45 to 70 mph). When I first got the truck (05 gmc 2500hd duramax diesel 2wd) I checked the odometer on the highway several times using 10 to 40 mile distances and it's about 0.02% high so I figure that is also "dead on" :-). Anyway, check your speedo and odometer to make sure your ecm knows what rear end gear and tires you have, so you can be sure you are starting with accurate speed, distance, and mileage data. With my bone stock truck I get about 21-22 mpg at 60 mph on the highway, but I lose about 2 mpg per 5 mph as I speed up from there, so drive as slowly as you have time for.