1991 Ford F-250 really bad gas mileage

I recently bought a 1991 ford f250 xlt lariat 4x4 with an aod transmission and figured I was going to get about 10mpg around town and 15mpg on the hiway. So this last time i filled up my tank by the way it has dual tanks I only got like 160 miles out of the front tank! Any suggestions on what I can do to see why I'm getting such poor gas mileage and how can I fix the problem.

Thanks