Thanks for the tip, Gary. I tried downloading Power Spec and following your instructions. In the end, I got what I believe is a strange answer. I tried multiple changes in the parameters and it always came back with the same answer.
I have an 1999 vintage ISB. When I plugged in the speed I wanted to go and asked for best mpg, it told me 2100 RPM and not lower than 1900 RPM. At 2100 RPM I'm doing about 73MPH and am getting around 9.0 MPG based on almost 5,000 miles of on the road experience with my rig.
If I drive around 1700 RPM, I'm around 56MPH and I'm getting around 11 mpg, give or take a little for the terrain. To get it up to 2100, I would have to be running most of the time in 4th gear. Somehow, that doesn't seem like what the engineers would have had in mind.
Mine is a lowly 5 speed Allison 1000 so I cannot believe that the transmission gearing is a factor.
Now if I had asked it about the power range, I could see the answers that I got but I asked about MPG.
2000 Georgie Boy Landau 36' DP
2005 Saturn Vue toad