My 2015 2500HD 6.0L LT Z71 4.10 6 speed auto Double cab with standard box, with snow plow prep package has gotten around 14.5 average for all vehicle miles to date (at just over 10,000 miles now) and has averaged around 17-18 on road trips. That was with E15 gas. it is now on E85 fuel and the average for all miles.. rural two lanes, gravel roads, off road, in town, highway, etc, is around 12.7. So lost about 2 mpg with the E85, but price wise I am still ahead as E85 is costing 2 cents a mile less to use even with the lower mpg. Granted, I don't drive it like a sports car and use traffic lights as my own cheap form of NHRA starting lights or try and beat everyone to the exit ramp on the freeway. But I don't drive down the road at 55 either. Just good, common sense driving I have learned over the years to get 25% better fuel economy from my semi trucks than the general trucking average mpg.
I did lower the back rake 2" with a set of McGaughy's shackles early on, and when empty, I don't run the stupid OEM recommended tire pressures of 60 PSI up front and 70 PSI in the back. I run about 50-55 all the way around when empty. I only inflate them up somewhat when I am going to load up with something. Contrary to what one would expect, by lowering the tire pressures, I picked up a little better fuel economy. The only thing I can figure out, is that with the OEM pressures, the rear wheel hop on gravel and bumpy roads was such that the anti-slip feature was continuously micro braking and other stuff so that fuel was being wasted. I have no definitive proof, just an assumption.
For commercial heavy truck tires, the tire makers usually have loading / pressure charts that show proper PSI for the tire loading. I contacted Michelin to provide one for the stock tires on my 2500HD, but they just punted the ball back to GM, who just has just blanket PSI recommendations of when the pickup is at max GVWR. The tires are rated for 80 PSI. My commercial semi tires are rated for 130 PSI, but recommended pressures for the max axle weight I haul is 95 PSI. I used similar differences to go with 50-55 PSI (when empty) on my pickup tires, and it seems to be working great. Tire wear is good, ride is better, and fuel economy a little better. for what it's worth.
