I'm running a bone stock 08 crew cab dually around for work, with a total vehicle weight of just over 12,000 lbs (weight box and bead bags in the bed). Drove to Flint and back last night and got 14 mpg with a regen in the middle. Not great, but not terrible. It rides like a caddy though... handles the weight llike nothing.
Special Thanks and Recommendations to;
PAC Racing Springs
Joint Clutch and Gear