I've been trying to understand why fuel economy decreases when the engine gets into higher RPMs. That is, if you put the pedal to the metal and quickly accelerate to 40 MPH, your engine operates much less efficiently than if you slowly accelerated to 40 MPH. Here's where it seems the efficiency loss happens. If you have a certain amount of fuel in a cylinder and ignite it, it puts out a certain amount of force on the piston. Now, if you double the amount of fuel and ignite it, it doesn't necessarily double the force on the piston. Where is the energy being lost? Is the resulting heat from the explosion conducting away, thus resulting in lower gas expansion? Or, is the energy being lost to mechanical friction somewhere?