Hehe you really like to do things the hard way! How's this -

On one tank of fuel:

Output energy = 5.3hrs * 3600 s/hr * 5500W = 105MJ
Input energy = 6.6gallons * 2.72kg/gallon * 44.4MJ/kg = 800MJ
Efficiency = 105/800 = 13%

Same as your answer of
5.5kW / 41.7kW = 13%

smile