3500 express ext loaded or empty no towing if i drive 64mph i know its hard, i get 22.50 thru 24.00 using ps in grey bottle now even with my bad injectors i am pulling 19 to 20 miles per gallon this is all non-stop hwy driving i usually go 500 to 550 between fill ups now, for city driving i drive easy and get about 15 to 18 miles per gallon. Remeber 64 mph is the sweet spot if u got a 3.73 rear