The Aveos can only go to 30 mpg, no more than that. You will not get it to better gas mileage because the car was not made for that. The newer cars and car models will have them going to 40 mpg or higher depending on what make and model you get.
What you can do in the meantime, however, is to get the fuel injection system replaced or cleaned out and that can be done at 30,000 or higher, and also the air filter to be replaced, as well as the car to get tuned up.
THAT is unacceptable as the only reason to drive one of these is for the gas mileage.
Having said that , does the thermostat function correctly, are your driving habits that bad ?
Is the engine running poorly , with a terrible miss .
Or is this engine somewhat out of time , but before I assume that I would suggest you get the vehicle scanned to determine what the engine may have as a list of trouble codes.
The engine light may not work as it does not in mine.
Depending on the result of a diagnosis of these potential issue a restricted exhaust would result in low vacuum and make for a fuel hungry engine.
In the end it may be a good idea to run from this one.
get the car tuned up and also have them replace the gas filter and the fuel injection system. The Aveos were made to only go up to less than 30 mpg, no more than that. What you are getting can also be very bad gas in your car, but also that your car needs to be tuned up seriously and the fuel injection system to be replaced or cleaned out. That can be done at 30,000 or so. Check with your manual and also call your dealership to tell them of your problem.