The inertia of the vehicle keeps the engine spinning. You could take the key out and throw it out the window and the engine would keep turning for as long as the car is in gear. Let's back up for a moment though. Let's say at steady-state, it shows 50mpg for you. When you lift off the gas and coast to a stop, are you saying that the SG instant MPG goes down to 45, 30, 20, 10, etc until you come to a stop? Or are you saying it jumps to 150mpg then decends to 130, 110, 80, 50, 30, until you come to a stop?