[MetroMPG]

You're in luck: there is an MPG gauge for non-OBD-II cars. Check out the MPGuino. If you can find a fuel injector wire and the VSS wire, you're in luck.

Can't beat instant/resettable trip average fuel consumption feedback for fine-tuning your game.

The coasting issue does sound very strange. Wild guess: maybe the car's feeding in a small amount of power when you lift off the accelerator at speed for smoother transitions? A MPGuino would confirm/bust that lame-brain theory.