how to increase mpg

Yeah, you ougtha be getting close to 23-25 easily driving on the highway at 60 mph.

Are you lead footing it? (just asking) It this an auto or manual trans? Do the brakes work well or is there any thing weird feeling in their action?

How does the car run in general while cruising? Smooth or rough? How does it idle? Smooth or rough?

Most likely issue (but not the only possible issue) is that the carb is flooding partially and dumping a lot of excess fuel in the intake. After warming up, take off the air cleaner, shut the engine off, and look down the carb throat and see if gas is dripping into the throat (and listen for it dripping too).

Check the plugs as above for fluffy carbon, which is a sign that there is excess fuel (running rich).

Have you rebuilt a carburetor, or do you have the skills and tools to think you can tackle that?