nevadanick wrote:
My point is what the manual states, its useless until you are at 75% of gvwr. And actually detrimental to your mpg.
Incorrect on both counts - from my 2012 Silverado owner's manual ...
"Tow/Haul is designed to be most effective when the vehicle and trailer combined weight is at least 75 percent of the vehicle's Gross Combined Weight Rating (GCWR)"Gross Combined Weight Rating, NOT the truck's Gross Vehicle Weight Rating.
I've been towing since 2006 with 3 different GM trucks, all with GM's ubiquitous 5.3L engine, all with a 3.42 axle ratio, the first 2 with the older, wide ratio 4-speed transmission, my current Silvy with the newer, narrow ratio 6-speed, and I've never seen any meaningful difference in fuel mileage whether Tow / Haul was engaged or not but all 3 certainly have towed far more easily with it on. :B