To calculate your car's fuel efficiency, write down the odometer or trip computer's mileage when filling the tank and the number of gallons added, then do the same over the next few fill-ups to calculate the average. FuelEconomy.gov shows official mpg estimates for each year, make and model.
While some modern cars offer real-time fuel efficiency readouts, less-expensive and older models require drivers to calculate mileage by hand. When you have around a half-tank of fuel, visit the gas station, and write down the number of gallons it takes to fill the tank. Then, reset the trip computer or make note of your car's total mileage as displayed on the primary odometer.
Upon the next visit to the gas station, making note of exactly how many gallons it takes to refill the tank to full shows the number of gallons burned during that period. Then, divide the number of miles you traveled as shown by your odometer by the number of gallons consumed. The resulting figure shows the average miles-per-gallon during that driving period. Repeating the process over the course of a few fill-ups gives a more accurate picture of what sort of mileage a vehicle is attaining in various driving conditions.
The U.S. Environmental Protection Agency tests and lists official fuel efficiency estimates for all consumer-market cars sold in the country each year. Those figures, searchable at FuelEconomy.gov, come organized by year, make, model, engine and transmission choice and give drivers a good general idea of a car's expected efficiency.