how to increase mpg

You need to diagnose what's wrong. It should be getting right about twice that. Do you have a factory service manual for the car? That's the first thing I would get. FACTORY, not Haynes or Chiltons or anything else. You can find them on Ebay. I would check things like tune up, and the general well being of the engine through a compression test. Check the air cleaner to make sure it is good and clean. Check the exhaust system to make sure it's not restricted by a collapsed pipe or stopped up muffler. Look at the plugs. Are they fresh? What color are they? If they are a dry sooty black, that indicates a rich mixture such as carburetor problems. Does it still have a point type ignition? They can be a source of contention if they are left unattended for long periods. They run very well if they are maintained, but over time if they are neglected, they can become out of adjustment since the points actually burn up slowly over a long period and the points rubbing block wears. All of this will cause changes in ignition timing and certainly cause bad mileage.