my dually has 410's and my best has been 16mpg. i have an air intake and a small super chip stock exhaust ( soon to change),have to keep it under 2g rpms to get that,and my frends,brother and dad have the same answer. i have a buddy with a cummins truck and he put 354 gears in his and likes it better. but a gear change is a big job and probably not in you plans. just my input hope it helps.