My next truck will be a 6 speed truck, but the current truck just rolled 35,000 miles so... I have a 2002 Silverado 2500 HD 6.0 and 4.10s. Fleet truck. No options, 6000 pounds with a full tank of gas, my son and I in it. My trailer is 7380 loaded on the scales and 32 feet long. Tows the trailer fine, the long wheel base helps(Crew Cab Long Bed). It is a dedicated tow vehicle. I have never calculated mileage. Just know it does better than my old 454. Tow all over Washington, Oregon and Idaho so we have few good size passes. Never missed a beat, just have to like 4000 RPM on the larger hills. Just let it sing, that is what it was designed to do!