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Originally Posted by EIA Energy Kids Page

BTU Content of Common Energy Units
1 barrel(42 gallons) of crude oil = 5,800,000 Btu

1 gallon of gasoline = 124,000 Btu

1 gallon of diesel fuel = 139,000 Btu

1 gallon of heating oil = 139,000 Btu

1 barrel of residual fuel oil = 6,287,000 Btu

1 cubic foot of natural gas = 1,031 Btu

1 gallon of propane = 91,000 Btu

1 short ton of coal = 20,754,000 Btu

1 kilowatthour of electricity = 3,412 Btu

Note that an engine optimized for propane could see gains, but running a gas engine on propane will most certainly see losses.