1992 F-150 bad mileage

I have this truck that came in my shop that gets about 6 miles per galllon. I drove it for a week and the customer is right you can almost see the gauge move. The thing runs and starts fine. Fuel psi is right on. I ran the scanner on it and the data that I can understand seems fine, I don't understand alot of the data but coolant temp, tps, o2, I know what they should read. I did notice the o2 likes to read on the rich side but not enough to set a code. Any Ideals or other parts of the data I should be looking at.