Do you know what kind of fuel economy your car gets?

To really know where you stand, you have to keep accurate records and do a simple calculation. The results might surprise you.
If you regularly compute your fuel efficiency you can find ways to save money on gas and know how to budget for the future. You can modify your driving habits and see what kind of difference that makes in the gas mileage. And, who knows, maybe you’ll find this record keeping fun. At the very least it will give you something to do while you’re waiting for the gas tank to fill up.
The first step is to buy a small notebook and keep it, along with a pen, in the door panel of your car. We suggest you create six columns in the notebook and label them in this order: date, odometer, trip, gallons or liters, MPG or KmPL, notes.
Date: This is the date you buy gas. This will help you budget for gas by showing you how often you fill up the tank.
Odometer: In this column, record the car’s odometer reading when you get gas.
Trip: The trip meter is usually in the same area as the odometer. It should be reset each time you get a full tank of gas. It will show how far you go between tanks of gas.
Gallons or Liters: This is the amount of gas required to refill the tank. You should lock the gas pump handle and let it shut off automatically. Then, do not top off the tank. True, each gas pump will shut off at a slightly different level. But there is no other way to verify that you have a full tank of gas. Later, you will be able to average the miles per gallon readings you get to find the lifetime average.
MPG or KmPL: This is the miles per gallon or kilometers per liter you get for a tank of gas. You will see how to calculate this later.
Notes: In this column you can make a note about any factors that might have affected the fuel mileage. Perhaps you left the car idling a long time. Maybe you did a lot of stop-and-go driving. Maybe you went into the mountains. This will explain any unusually high or low readings that you get.

After you set up your notebook, get a full tank of gas and drive. When the needle gets to about a quarter tank, fill up again. Now it’s time to see what kind of gas mileage you got on this tank of gas. (Note: your mileage from tank to tank will vary. You should look for a pattern before concluding what kind of mileage you get. Also, you will eventually want to average your individual readings to get a "lifetime" average mileage.)

Calculating Gas Mileage
This is an easy calculation. In fact, if the readings you get are nearly round numbers, you can do this in your head. For example, if you fill up your tank and it takes 20 gallons of gas and you drive 400 miles, then you divide 400 by 20 and get 20 miles per gallon. However, often you will drive 261.8 miles and buy 12 gallons of gas. What do you do about this? Well, you can turn to another page of the note book and do long division. Or, most cell phones these days have a calculator in them. Plug in the numbers and you see that this is 21.75 mpg. Make sure you record this figure in your notebook.
After a few months of calculating your fuel efficiency, your MPG column might look like this: 19, 18, 22, 25, 19, 27. You will want to know the average of all these figures. To determine the average, add all these numbers together and divide by the number of figures you are averaging. In this case you would total the numbers and get 130. Now, divide 130 by the number of figures you are averaging which is six. This will give you a lifetime average of 21.6 mpg.