I bought this car used about a month ago, so I saw no DECREASE, but I figured I'd add to this category.

The reason I bought this car, was because it was a mid-size 4 cylinder, that got good fuel economy ratings. Unfortunately, there's no easy way to determine fuel economy during a test drive.

I'm averaging 18 mpg...

I'm not a terribly aggressive driver, there's no leaks or anything... The only thing I can think of that might be hurting fuel economy is the wheels that the prior owner put on. They're 16-inch rims. I never thought when buying the car, that a 1-inch increase would hurt fuel efficiency so much.

Does anybody know what could be causing this dismal fuel economy?

I bought my car new in January 2000 in California. Since then I have run the heck out of it. Every recall, brought it in to the dealer for a fix. Transmission got the additional cooler and transmission doesn't burn oil or slip. Seat belt buckle fixed. Alternator kept on blowing up battery until I put in a 3rd party one. But I recently had just an oil change (trans and engine) and tune-up done and new tires put on, suddenly my gas mileage went from approx 20 (city) and 40 (hwy) to 10 and 15 respectively. The car surges, RPMs run from 1500 to 3000, and the check engine light comes on and off. I could tell the mechanic was having problems with it, when he kept on getting "consultations" with other mechanics. They gave me the car back and said it is just normal "burn-in" for the plugs they used. WTF?