I figure every time I fill up. Depending on the distance and the way I am driving, I run between 35 and 38 miles per gallon (I drive like a friggin grandma).
I am going to do the real test of my fas mileage in June, I am taking a trip from Seattle to Martinez, CA. It's about 750 miles or so. My aveo is all stock, so I will be able to get a pretty accurate number. I mean, on the 5 I can set the car at 70 and roll on till empty. The mountains may cause a drop though. We will just have to see. If nothing else, it will be a pretty good "real world" measure.