Why is the mileage getting worse

The mileage keeps getting worse. The transmission shifts properly and I keep it as well as the rest of the vehicle maintained. Several years ago the catalytic was replaced under warranty. I have recently changed the fuel pressure regulator, the hub bearings and the cooling fan assembly. The check engine light has not come on, just the mileage has dropped from the average of 23-25 mpg to now at 20-21. I don't know what else to look at but I don't want to buy another GM cat as it costs over 400 dollars.