I towed a 15,000 lb trailer behind a Duramax without an exhaust brake. going down a long steep grade I was able to keep my speed down to 42 mph without the use of the wheel brakes. I Had to slow down enough to get the transmission to drop to low gear. In 1st gear the engine revved to 5200 rpm. Second gear was a run away. A gas engine likely could have held that same trailer back at 55 mph and 4000 rpm in second gear. With an exhaust brake on these newer diesels you can pretty much just hit the tow haul button, turn the exhaust brake on and relax.