I have a 4 x 4 2500, no preformance mods. I do have larger tires (265/75 - 16 BFG AT/KO) and the 4.10 diff. I am getting about 9 mpg around town (Las Vegas - stop and go to work and lots of heat - AC on full). On the road not pulling anything going about 75 mph I get about 11 mpg. I pulled a 20' camp trailer to the beach in CA - lots of head wind and hills - no problem pulling 65 to 75. Gas mileage was about 8 mpg. I am running Mobile 1 (5w - 30) and only have about 5,000 miles on my truck. I love the big block, but it is thirsty. Cheers - Jim