My t stat was broken for a while, the small metal frame was actually broken so it was always open. In the colder months, my truck would never warm up above 140-150*. Mileage increased greatly when I put in a new t-stat. When I was flushing the system with water I realized how much better water transfers heat that glycol water mix.

Recap: I know for a fact that my mileage is much better at 190* Tstat than a broken open t-stat at 140-150*. I know for a fact that my mileage goes up as the outside air temp goes up. I know for a fact that my mileage has improved moving from Utah 5000 ft elevation to Vegas 2200 ft elevation.

I don't argue that a truck running at 220* will not get better mileage, I'm just saying that a zero water system will definitely run hotter in overall temp. Now if the motor can run fine at 250* with the fancy coolant, and not boil over or cook the water pump, great! I would guess that mileage will improve the hotter the motor runs up until internal friction increases due to swelled piston slugs/thinner oil films.