Fill your tank up and take note of the odometer reading before you leave the gas station.
Drive around for awhile until the gas gauge gets down fairly low. Try to make it an easily measurable point, such as 1/4 or 1/8 of a tank.
Refill the tank. Take note of the new odometer reading and the amount of gas you've put in on this trip to the gas station.
Figure out the difference between the two odometer readings you've taken by subtracting the first reading from the second. Divide this number by the number of gallons of gas you've just put in your car. For instance, if you drive 300 miles and you put 15 gallons of gas in your car, your car gets 20 miles per gallon.
Repeat this process several times, including a few times on the highway. Add up all miles per gallon averages and divide them by the number of times you've completed the process. This is your average MPG rating. For instance, if you did the experiment seven times, with a rating of 20 MPG, 22 MPG, 25 MPG, 19 MPG, 27 MPG, 32 MPG and 33 MPG, the average would be 25.4 MPG.