Adding the GearVendors to my '89 GMC 454 Dually, TH400 w/4.10s, picked up highway MPG by 3-4MPG. 5500 lbs empty, which it rarely is, usual roadgoing weight is more like 6000. I generally drive it casually, but when not towing, it manages to find 70 or so. The lower RPM is pretty nice too.