There are several factors such as drag, power to weight ratio, gearing, etc. With the V8, I'm assuming such cars benefit with taller gearing. Our cars have shorter gearing for faster acceleration, but it sacrifices a bit of mileage. I'm only speaking off the top of my head so don't quote me lol.
Anyhow, I'm pushing at least 450 whp and my average combined MPG is around 22.
