My was getting easily 29 mpg. Now I am getting 21 mpg. Any suggestions for troubleshooting this matter? Its a 2002 avalon.

Well, if you've flogged the car half to death in two years, you might want to consider a tune up...even if you haven't flogged it, you should still consider a tune up if one hasn't been done since the date of manufacture. Also, there might not even be anything wrong with your car at all, it could just simply be alternating patterns in your driving, changes in climate, and RPM ranges throughout driving. There are lots of things to factor into the gas mileage factor before blaming something wrong with your car.

You usually get lower mpg during cold weather. Check your tires pressures too?

clean throttle valve

Try higher octane gasoline.