I have a 1997 f150 that I bought used in July with 92,000 miles on it. When I first got it I was getting 15-16mpg. Now I am only getting 11-12. Since then I have added more weight for the winter weather (About 400 lbs). I also had the check engine light come on but I put fuel injector cleaner in it and its off now. Could low gas mileage be because of the Oxygen sensors? What else would cause it to drop that low?

 

It could just be winter (oxygenated) gasoline. I typically get about 2 miles less per gallon starting in November.

 

Yep, I also lose about 2 MPG during the winter on my '97.
If you do alot of city driving I imagine it is worse.

In your case I would wait until it gets warm again and see if your mileage is still lower than normal, also keep an eye out of the check engine light.
You might try going to AutoZone or a similar place and seeing if they can still pull any codes from it.

 

Have you checked the plugs ? what shape are they in and the air cleaner also. When I get a used vel I check these when I get it home and find the air cleaner is dirty and the plugs need to be changed out,also the brake pads and shoes.