here's a way to find out

fill up all the way, reset ur mileage counter, then wait untill the tank is half way empty.

then go back to gas station, write the down how many u drove, then fill it back up and divided the mileage u drove by the amount of gallons u put in.

another thing u gotta watch out for is people mixin water w/ their gasoline, they cought a few gas station owners doing that here in houston.

earlier this year, my car was givin me 11mpg and after changin out the o2 sensor (even though i didnt have any CEL), it went back up to aournd 17-18mpg, and thats where it stays for me.