Is there a correlation between the MPG of a vehicle and RPMs?”
Yes, an engine will consume more gas at higher RPMs, even when it is lightly loaded. Thus it will cost you a bit more to drive at 35 MPH in third gear (assuming a manual transmission) than it will to drive the same speed in fourth, even though the load on the engine due to wind drag, vehicle weight, and rolling resistance, is the same. The faster the engine turns, the more work it has to do inside of itself, and more is lost through the transmission. For optimal gas mileage, you want to drive in the highest gear you can maintain without lugging the engine, and downshift only when you need power to accelerate.
