City mileage is a crap shoot. What if you live in a place with a traffic light on every corner? How can you compare that to some guy who drives expressways in the city? The answer is you can't! City mileage listed on window stickers is a myth. Its a test done under closed conditions for the EPA. Okay now that you understand...
Highway mileage is a better indication of gas mileage. Set the trip mileage then fill the tank, when the tank is near empty from driving, fill up. The gallons replaced in the tank is the number of interest. Check the trip mileage. Divide the trip mileage by replaced gallons (including any fractions) and the result is gas mileage. This will still vary with altitude, hills, temperature and a number of other variables. Do this many times and you'll be able to calculate an average highway mileage. Whatever mileage you get tells you about your driving habits. It isn't meant to compare with another car because that driver's habits may be entirely different from yours.
City mileage works the same way. You have to check and log the mileage many times to build up an average. Some people keep a little log book in their glove compartment and log it every fill up. Some cars have a trip computer that does this for you. My Explorer has one of these and the mileage is about 17.5mpg (total average since it was originally purchased). I never erase the total because if I see it suddenly diminishing, I know something needs attention. The Explorer gets about 23mpg on the highway.
My '01 GT gets 20 mpg in a mix of town and country driving. It gets up to 26 mpg on level highways (no mountains). Its an automatic. From this you can assume I don't race. Its true, I don't race for any reason because I'm past the age where racing is possible. I don't have the reaction time I used to, so its futile to think I can do as well as a 20 year old. Cruising is more fun anyway.