For those of us who own oil burners, aka diesels, we use a pyrometer which measures exhaust gas temperature. The higher the number the more fuel is being consumed (BTU's are being generated)thus giving you a good idea of the load being placed on the engine. Our EGT's run between 750 & 850 degrees at 65 MPH cruise on flat land with no wind. At 750 degrees we average 8.5 mpg and @ 850 degrees we average 7.5 mpg. The single most contributing factor to mpg is speed, like Sammy Hagar proclaimed, I can't drive 55 but I did compromise at 65, down from 75. While I do accept the fact that ultimately it takes what it takes to move our rigs down the road, driver discipline is the single most contributing factor in our cost per mile to operate our vehicles.

Amen to that. My smaller engine (than billyt's) runs around 550°F at cruise, ~3 psi boost.

I'd add to billyt's advice that the boost gauge is also a good indicator, as boost will rise faster than EGT (but only slightly faster).

Comparing boost, EGT, and coolant temp - when they get out of their usual harmony - can indicate tme to do something, like change air filter or clean the radiator.

One device can definitely increase your fuel economy: The right foot.

10-4 on that Rusty!

No turbo on my bus, so no boost gauge and also no pyrometer. I just watch my exhaust. If i can see black smoke i know i am wasting fuel, so i let up on the pedal a bit or downshift if needed.

My Detroit Series 60 comes with a thing called ProDriver. It measures fuel consumption constantly. I believe it was designed to track efficiency of multiple drivers in 18 wheelers as it has ability to put in many driver names and compare trips. It also tracks idle time which is a big factor in mileage in urban settings.

I've had the vacuum type gauges on my passenger vehicles, but this is a first for me on an RV. It has a great influence on how I drive as it appeals to my competitive nature to try to beat my last run economy