with my edge in stage 3, I am getting about 15-16 highway... but my mileage has SUCKED since day 1. After talking to several techs, 05 was not the best year for mileage, the trucks are all over the board. One GM Commercial tech even referred to the 05's as the "black hole" year for the duramax. very comforting. Pulling a 24' enclosed car trailer i get 10-11 at about 80MPH.