what would cause fuel economy to go down on 1993 Volkswagen Jetta

My fuel economy seems like it is a lot less than it should be. a 4cyl jetta like mine should get at least 350 miles on a full tank but right now i'm only getting about 280 miles. I am going to try getting my spark plugs replaced and my tires rotated, but is there anything else I might want to try besides that?

Asked by for the 1993 Volkswagen Jetta
New air filter and fuel injector cleaner could help.
thanks! i'll try that, and since i was contemplating on upgrading my air filter to a short cam eventually, this helps influence my decision a little bit :)