I dont automatically use tow haul. At times it's better to just run in 5th because my main concern is to minimize the trans shifting in and out. It's RV'ing so I dont pay attention to mpg or worry about differences in fuel prices. I have used vac gauges to diagnose engine problems years ago but I would focus on not having constant shifts going on and get the feel of where it's on the pipe and be able to anticipate upcoming hills and let it roll.
I have driven my diesel with an eye on the boost gauge just to minimize shifts but there is enough other stuff going on that I dont do it any more. Fuel use is what it is so I focus on minimizing wear on my trans parts and not lugging the engine.
