I agree with Kit on this. Not being an engineer, if the slant develops 100 HP @4500 rpm, you don't usually drive around at 4500 rpm. Lets say you drive at 1500 rpm and only develop 1/3 the max HP, or about 30.
In your equation, 30 HP would use 15 # of gas over 60 miles, or about 2 1/2 gal...