It all follows simple math. Efficient gasoline engines will burn in the range of 0.4 pounds per hour per horsepower, give or take a little.
If your engine makes 300 HP to crest that hill because you put your foot into it, it will burn approximately 120 pounds per hour -- on instantaneous basis.
And 120 pounds per hour of gasoline is 20 gallons per hour.
If your motorhome is only doing 40 MPH, then that 20 gallons per hour yields 2 MPG.
Basically, any modern gasoline engine that has to make 300 HP to maintain 40 MPH going up a hill won't do much better than 2 MPG. Some could do much worse under those power conditions, but upside is limited.
With present designs not many gasoline truck-ready engines can do much better than 0.4 pounds per hour per horsepower. And the Ford V10 isn't far from that.
The upcoming Ford 6.2L V8 should do a little better, but even if it is 10% more efficient (and I doubt it will be at full throttle), what does it really mean? That instead of getting 2 MPG up a hill we will get 2.2 MPG?
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