I've been using an inexpensive Garmin Nuvi 205 GPS in my 93 Swift for the last couple weeks. It has a program called EcoRoute that routes for the best gas mileage based on speed limits, stops, acceleration data, etc.. It also has an Eco Challenge program that displays an icon in the screen with a number between 0 and 100 and a red, yellow, or green marker. It monitors acceleration, speed, braking, giving you instant feedback on how efficiently you're driving, projected gas mileage, carbon footprint, and fuel cost. Works amazingly well for a device that isn't connected to the cars computer or sensors. So far I've found it to be within 2% on predicted gas mileage.
I think it's a good option for older cars that can't use the Scangage and a lot cheaper and easier to install.
: a psychological condition or a relationship in which a person is controlled or manipulated by another who is affected with a pathological condition (as an addiction to alcohol or heroin) ; broadly : dependence on the needs of or control by another