Town Car :: Gas Mileage Is Too Low On 2002 Lincoln

I recently purchased a 2002 Lincoln Town Car with 204,000 miles but it runs like a champ. The Problem Is It burns too much gas i filled the tank up and2 days later I'm at a 1/4 tank without driving very much. My fuel economy states that i'm getting 8.5 miles a gallon this number goes up and down. I changed the oil and air filter still having the same problem. I need to know what I can Do to fix this I have heard the plugs and possibly the coils but I would like to know if there is more or perhaps something else I will be doing the work mostly myself.