Can you be more specific? Are you talking about what the car reports on the trip computer or the EPA estimates on the Monroney (window) sticker?
it considers everything. total gas used divided by total miles driven. then it adds a couple mpg's to make you feel good.
Go to fuelly.com and create an identity for your car and just enter the odometer reading every time you fill up. It gives you best/latest/worst/average figures as well as graphs. Plus you can compare your driving to how others are doing. Doing this over several tanks is much better than trying to use the dashboard readouts.
