I've always used 18 HP per hr per gallon for diesel and 14.5 for unleaded gas. This assumes internal combustion engines, 4 stroke, normally aspirated. I did some research on small engine fuel consumption and it goes like this:
|Natural gas||9-11 hp-hr/ccf|
|LP (propane)||9-11 hp-hr/gal|
The size of the engine, operating temperature, RPM, and Load all have an effect on the efficiency of energy conversion.