If you have 3 cars moving down the highway – a 4, 6, and 8 cylinder model of the same car – they will all require the same amount of power be generated to keep them moving without speeding up or slowing down. There are two main things that impact the gas mileage. Differences will be mostly due to the ease with which changes in the fuel mixture can be achieved. IE: you have a set amount to move the gas pedal, but in the v8 car moving it x distance will yield more power made – and thus more fuel used – than you would find in the other models. That is entirely because the driver is choosing to accelerate at a higher rate. If both cars accelerated at the same rate, or didn’t accelerate at all, the difference in economy would come from within the engines. There are varying efficiencies with which the combustion event happens, and that will predict how well the fuel is used.