Yeah, the best thing to do it to fill it up, reset the ODO, then wait to it nears empty, fill it up, then take the miles driven and divide the amount of gallons you used to fill up to being full again, the fuel gauge itself is not very good on there cars. In heavy traffic and heat, you should see 22-23mpg city at worst, combined, 27mpg is a very fair number if not higher. The auto tranny on the aveo's is not very "good", if you can get to over 42-42 mph then you will see a jump in mpg's. Also, if you see lower mpgs then what I have said, you may want tocheck your tires, spark plugs, air filter etc....

Welcome to the forum btw! We are here to help you (go on 4chan at all?)