Sebas, if you want to know it exactly, look in the specs (technical datas) of the manufacturer. In Europe the specific fuel consumption of each engine is given by the manufacturer in g_kwh (gram per kilowatt hour). That means for each kw (0.74kw=1hp) an enigne produces in an hour it uses so many grams (1000 gram = 2.2 lbs) of fuel. The engine with the lowest g_kwh is the most efficient one. The manufacturers have to guarranty their numbers by law.