MumbleVille

This is not a list of ways to improve your fuel economy. This is also not a discussion of the disclaimer printed underneath EPA mileage estimates (you know, the one that says “your mileage may vary”). If you want information on that controversy, look here.

This is an arithmetic class (maths, for those whose native language is British English). It is a primer on how to calculate your gas mileage. I’ve grown weary of people talking about how it costs $100 to fill the tank on their {insert name of vehicle here}. That number is meaningless!! It’s meaningless because the listener has no idea how many gallons it takes to fill the tank and also no idea how far you’ve driven since your last visit to the local AM/PM (or Starvin’ Marvin’s).

Determining your vehicle’s gas mileage is very simple in this age of trip odometers¹. Here’s how you do it:

Step 1: Fill your fuel tank. Let the pump run until it shuts off automatically. Do not top off, because that adds to air pollution and wastes a small amount of fuel.

Step 2: Sit down in the driver’s seat and zero out the trip odometer. Check your owner’s manual for instructions.

Step 3: Drive your vehicle until you think you need to fill the tank again. Fill the tank, letting the pump run until it shuts off automatically. Get a receipt that shows the number of gallons².

Step 4: Check your trip odometer and record (or remember) the number.

You now have two numbers: The distance traveled and the number of gallons consumed. To calculate fuel economy, divide the first number by the second.

MPG = distance traveled ÷ gallons consumed

This number is reported (in the USA) as “miles per gallon.”

Example

1) It took 27.7 gallons to fill your Hummer II. You’ve driven 293.5 miles since your last fill-up. Your gas mileage is calculated using the formula

MPG = 293.5 ÷ 27.7
Your fuel economy = 10.6 MPG (miles per gallon)

2) It took 13.2 gallons to fill your Honda Civic Hybrid. You’ve driven 551.8 miles since your last fill-up. Your gas mileage is calculated using the formula

MPG = 551.8 ÷ 13.2
Your fuel economy is 41.8 MPG (miles per gallon).

Since your results will vary (would the EPA lie to you? well, yes, they would) with driving conditions, you’ll probably want to calculate this number over several fill-ups to get a good feeling for your mileage. Most drivers get lower mileage in their daily commutes as compared to long highway drives – that’s why our friends with the EPA report both estimated City and Highway mileage.

By Way of Illustration

The cost of filling the Hummer is about twice that of filling the Honda, but the Civic’s mileage is nearly four times that of the Hummer.

If You Live in the Metric World

Many countries that use the metric system report fuel consumption in liters per 100km instead of fuel economy in terms of mileage. Lots of people in the US are a little hazy on what a kilometer is, though most now know what a liter looks like, thanks to two- and three-liter bottles of soda. For all of us, here’s a handy conversion factor:

MPG = 235.2 ÷ (liters per 100km)

Example:

Your Bentley consumes 21 liters of petrol per 100km of driving. To convert to miles per gallon:

MPG = 235.2 ÷ 21
Your fuel economy is about 11 miles per gallon.

Should you need to convert miles per gallon to liters per 100km, divide that same magic number (235.2) by the fuel economy in MPG. The Honda (above) consumes 5.63 liters per 100km, the Hummer, 22.2 liters per 100km.

There:

Now you no longer have an excuse to try to express your gas mileage in “dollars per fill-up”!

¹You say you don’t have a trip odometer? Don’t have a cow: just scribble the odometer mileage on the receipt each time you fill up and do the subtraction. Use a calculator if you need to.
² It’s important to use the actual number of gallons it took to fill the tank. Don’t look the capacity of the tank up in the owner’s manual and use that, because unless you rolled into the station just as your car died from lack of fuel, the tank wasn’t empty. In fact, almost all cars still have about enough fuel to travel thirty or forty miles in the tank when the needle reads “E.”

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