If everthing is stock, a 600cc will always get better mileage than 954, with the same rider, same driving conditions. Put a three hundred pound guy on a 600 and it would have to work overtime getting close to a 954 mileage. Please dont compare a 12 cylinder to a civic. 600cc = 0.6 liters, 954 = 0.954 liters a 3 to 2 ratio. Civic = 1.5 liters, Lamborgini = 6.5 liter 6 to 1 ratio. Load the Civic down with more weight than it can handle and mileage will drop closer...CLOSER... to that of a Lamborghini, but never the same. The heavier the rider, the worse the mileage, the less affected the larger engine, there you have it.