Why would my mpg be going down?

What is your question?

Have 124,000 miles on the car. Was getting 25 hwy mpg & 22 mpg around town. Replaced the timing belt 5,000 miles ago after it slipped a couple of teeth. Mileage has now been in the 20-22 mpg. Just replaced the spark plugs and wires, put in a new air filter and mileage was 19 mpg the last 1/2 tank. There are no warning lights, it starts fine and runs smooth (rpms seem to be higher than I remember from before). The only scheduled maintenance I haven't done yet is to replace the fuel filter. Would that cause the mileage to drop without other symptoms? Any suggestions?