Examples of computing miles per gallon using weighted averages.
With the price of gasoline setting new records every month, it is important to know how efficient a vehicle is when it comes to using gas. Computing miles per gallon (MPG) is a simple mathematical calculation.
The examples used will be similar under the metric system, substituting kilometers for miles and liters for gallons.
Computing Miles per Gallon
Fill the gas tank and write down the mileage. If the car has a trip odometer, reset it to zero. When filling the tank, do not “top it off”, or keep pumping after the pump stops initially.
Topping off the tank can result in spillage, which results in an inaccurate measurement and the risk of fire.
Drive normally until the tank is less than one half full. An unusual amount of freeway driving will result in a result that is too high. If there is an excessive amount of stop and go driving, or if the car idles in a traffic jam for long periods, the MPG will show as too low.
Allowing the tank to get to less than one half will smooth out most variation and give you a more accurate reading. Calculating the MPG after using just a few gallons makes the result less precise.
Fill the tank in the same manner as before, resisting the urge to top it off. Write down the mileage from the odometer or the trip odometer if you used it, and the number of gallons used to the tenth of a gallon, (e.g. 10.4).
Subtract the first mileage number from the latest. Then divide by the number of gallons.
For example:
New mileage of 10,400 minus old mileage of 9,900 = 500 miles divided by 20 gallons used = 25 miles per gallon.
Figuring a Weighted Average
Multiple measurements over a several fill-ups averaged together will give you a more accurate result.
Do the calculation over two fill-ups. In order to get an accurate reading, you cannot just add the results together and divide by two. Because the amount of gasoline used will be different each fill-up, this needs to be a weighted average.
An example using two fill-ups:
500 miles divided by 20 gallons = 25 MPG
200 miles divided by 10 gallons = 20 MPG
A simple average would yield the answer of 22.5, but the correct answer is 700 miles divided by 30 gallons, which equals 23.3.
Using Miles per Gallon
Do not be surprised if your miles per gallon vary greatly from the published miles for your car. If the MPG is very different, recheck the calculation.
If it still well under the estimate, then it may be time to perform maintenance such as new tires or a tune-up.
