When I towed 8000 lbs from OH to AZ and back, with a 06 DMax, my MPG ranged anywhere from 10 and 16.5 hand calculated. My DIC (computer mpg readout) always reads a little low. It is very obvious that grade, wind, and road conditions are some of the major variables in mpg performance. I can see my DIC readout change in real time whenever any of these variables starts changing. I even see my mileage increase after transitioning from a noisy concrete stretch of interstate onto a velvety smooth asphalt surface.
Since I have learned to trust my DIC, I use it to optimize my mpg by looking at the results instead of the inputs. I know my best MPG is somewhere around 60, so I reset my DIC and move up and/or down from 60 (using cruse control +/-) to pinpoint my best MPG performance. If road conditions (vibration), elevation (GPS), or wind (trees and grass) change, I reset the DIC, and hunt out the optimal point using the cruse control again. I normally don't have anankastic personality disorder, so I only do this when I get bored.
But, I must admit, a couple new gauges in the old truck would be very cool.