91 Tercel Gas Problem

Hello, I have a 91 Tercel and everything is fine with it except that the gas consumption has gone up dramatically over the past few months, it's burning nearly double what it used to. My guess is that it might be a dirty fuel injector, dirty air filter or spark plugs. I've checked the distributor cap and it fires fine and the PCV valve is clean and everything else looks good as well. Please advise as to what the most likely cause might be, I'd rather not spend money fixing things at random. Thank you.