I think your equivalence on propane is way off.
1 gallon of propane = 4.2 pounds @ 60F.
1 gallon of propane = 84,300 btus
1 gallon of gasoline = 114,100 btus
so a 20 pound propane tank is 20/4.2 = 4.76 gallons
equivalent to 4.76 X (84300/114100) = 3.5 gallons gasoline
Thanks to both of you. I knew smarter folks than I would find the holes. I appreciate the sharp eyes.