One USA gallon of regular unleaded gas is equal to about 49 horsepower in heat units. In practical terms, about 65% of that is lost when it's burned through an engine.
An automotive engine will produce about 15 HP hours on a gallon of gas. An automotive diesel will produce about 20 HP Hrs. on one US gallon of #1 diesel. That's 20 HP for a period of one hour.
The larger the engine, the better the HP hour rating. A 500 HP stationary diesel can achieve up to 22 HP Hrs. per gallon (US) of diesel. HP hours are generally used commercial and agricultural engine applications.
efficiency is different for each engine. But there is only so many HP you can make per gallon.
