Tuesday, June 22, 2010

How to Calculate Your Car's Fuel Efficiency (MPG)

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 GH₵.10 rise in gas prices will affect your budget, or how getting a car with better MPG will lower your monthly costs.

1. Go to the gas station and fill up the fuel tank.

Take note of the price you pay

2. Record the mileage, before even pulling away from the pump.

We will call this Mileage A.

3. Drive normally until the tank is less than half full.

4. 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.

5. Record the mileage again, just like before.

We will call this Mileage B.

6. Subtract Mileage A from Mileage B.

This will give you the number of miles you drove since your last fill-up.

7. Divide your answer by the number of gallons it took to fill up your tank.

This will give you your car's MPG.


  • The vast majority of cars will be equipped with a trip odometer - this is a gauge that counts mileage and can be reset. This gauge is in addition to the regular odometer, which counts the number of miles a car is driven overall. One can use this to count mileage. Divide the total miles run on a full tank of fuel by the capacity of the fuel tank to obtain the mileage of the car.
  • 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.


  • 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.
  • In other countries, the equivalent is in kilometres per liter (km/l). In the United Kingdom, fuel is sold by the liter but fuel consumption is given in miles per gallon. Thus liters per 100 kilometres is used alongside miles per imperial gallon. However, in the United Kingdom, fuel efficiency is very hard to calculate, because of their 1965 metrication, and is ongoing for 45 years already. Britain is notable for having the 45 year muddle with Imperial (English/US) and metric measurement.

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