Welcome to EM,
I don't think that any vehicles prior to 1996 have the OBD II data plug that you need for an MPG meter?
You can check any threads by the owner of that car:
Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Profile: wagonman76
...and see what mods they have done and/or how they drive to get the MPG.
Tank by tank MPG is the most accurate way to go, and you can check the calibration of your odometer, too.