What kind of car is it? Specifically, what model year? Cars before 1996 will most likely not work with the ScanGauge at all.
If your car is OBD-2 compliant, the the SG is a good idea. You can set up gauges on it for instantaneous MPG, trip MPG, and load value in addition to the coolant temp one. Those can help you change your driving to give you even better economy...
-soD