As gas prices rise, fuel efficiency is becoming more and more of a critical factor. Knowing your car's MPG (that is, how many miles it gets per gallon) can help you determine if it's is a gas guzzler that's eating up your wallet as well. Once you figure out the MPG, you can do many useful things, like calculate how much a $.10 rise in gas prices will affect your budget, or how getting a car with better MPG will lower your monthly costs.

[edit] Steps

Take note of the price you pay

Go to the gas station and fill up the fuel tank.
Record the mileage, before even pulling away from the pump. We will call this Mileage A.
Drive normally until the tank is less than half full.
Fill up the tank again (preferably at the same station using the same pump as pumps may be calibrated differently). This time, pay attention to how many gallons it takes to fill up the tank. This is usually shown at the pump.
Record the mileage again, just like before. We will call this Mileage B.
Subtract Mileage A from Mileage B. This will give you the number of miles you drove since your last fill-up.
Divide your answer by the number of gallons it took to fill up your tank. This will give you your car's MPG.

[edit] Tips

The higher the MPG, the more efficient your car is, and the cheaper it'll be to keep it fueled.
To determine how a change in gas prices will affect your budget, take the number of miles you expect to drive in a week (or a month, or a year) and divide it by your MPG. Then multiply that answer by the price of gas per gallon. By plugging in different prices, you'll see how much more - or less - you end up paying per week (or per month, or per year).
Try calculating your MPG more than once to get a more accurate measurement. If you did more highway driving than normal, then your MPG will be a little higher. On the flip side, if you did a little extra city (stop and go) driving, your MPG will be lower.
You can use the MPG to experiment with ways to increase fuel efficiency. For example, if you normally drive at an average of 70 MPH, then after calculating your MPG, try driving at 55 MPH and measure your MPG again - you'll probably see it go up.

[edit] Warnings

Mileage will vary with different driving patterns, the less braking and acceleration will lead to better mileage. You will see higher mileage when taking highway trips than you will after a week of driving back and forth to work on city streets.

[edit] Related wikiHows

[edit] Sources and Citations

FuelEconomy.gov - Find out the EPA Fuel Economy ratings for new and used vehicles, and learn more about how you can improve your MPG.
Stop Static Report - A report issued by the Petroleum Equipment Institute regarding the recent rise in static-related gas station fires.
GasDandy - GasDandy is an easy-to-use tool that tracks a vehicle’s mileage and maintenance information, providing data that can be used for both business and personal purposes. By making these figures readily available, the program also gives the consumer the opportunity to save money and to proactively identify problems that can shorten the life of their vehicle(s). Download a free trial version of GasDandy today.