If you look up your car's fuel economy on the Environmental Protection Agency's Web site, you can find the official rating for your car -- as long as it was built after 1984. But the official rating probably isn't your very own actual fuel economy. The EPA puts cars on rollers in a lab and runs them through a very specific set of tests. You, on the other hand, drive like a maniac. Don't deny it.
There are two ways to gauge your vehicle's fuel economy: miles per gallon (mpg), which is probably the most common way to phrase it in the good ol' US of A, and gallons per 100 miles (g/100m) which is all newfangled and Euro-style -- sort of like the Daft Punk of fuel economy. It's also a much easier way to compare fuel consumption between two different vehicles. Here's how to calculate either.
Miles per Gallon:
Next time you fill the tank, note the mileage on the odometer, or simply reset the trip odometer to zero.
When you fill the tank again, note how many miles you've driven. Let's say you're in a Lamborghini Murcielago and you drove 300 miles. Or more likely, you're in a Geo Metro with a three-cylinder engine and drove 400 miles.
Now look at the receipt (or the gas pump, if you're still at the station and reading this how-to on a mobile device) to see how many gallons of fuel you put in the tank. The Lambo holds 24 gallons and the Metro holds 10.6 gallons; we'll assume you drove until it was completely empty.
The formula is right there in the terminology: miles per gallon. Take the miles driven in the Lamborghini (300) and divide by the number of gallons you added to the tank (24) to get the miles per gallon. That's 12.5 mpg, by the way. The Geo Metro example works out as 400 miles divided by 10.6 gallons = 37.7 mpg.
Bam! You got your miles per gallons right there, math ninjas. And now that you know how to calculate your car's specific mpg, you can reverse that business and find your gallons per hundred miles.
Gallons per 100 Miles:
First, figure out how many gallons of gas you use in one mile by finding the reciprocal of your car's mpg. You were worried, weren't you? Thought you'd have to remember what the heck a reciprocal was all on your own? Relax. It's just dividing one by the number in question -- in this case, the number in question is your car's miles per gallon. So, using the Lamborghini example above, it's 1 divided by 12.5 and for the Geo Metro, it's 1 divided by 37.7.
The answer is your car's gallons per mile. So, that's .08 for the Lamborghini and .026 for the Metro.
Multiply that number by 100, and you've just found your gallons per one hundred miles! It takes 8 gallons to drive 100 miles in the Murcielago and 2.6 gallons to drive the same distance in the Metro.
Quick Tip: The EPA is adopting the gallons per 100 miles model, so you're going to see this number listed near the miles per gallon rating on every window sticker if you're car shopping in the near future.
Author's Note: How to Calculate Your Car's Fuel Consumption
I realized in writing this that I had never calculated my car's fuel consumption. I know that I fill up every 250 miles or so, and that at my last fill-up, I purchased 12.38 gallons of gas (the receipt was still stuffed in the console cubbyhole). So that's 250 miles divided by 12.38 gallons, for a kind of sad 20.19 mpg average. According to the EPA, the combined score for my car was supposed to be 21 mpg, so I'm a little low. They also have the average user's mpg, which is 26.9 mpg. I'm way below that, too, which says a lot about my driving -- maybe.
They didn't rate gallons per 100 miles in 2005, so I'm on my own here. But if I take the reciprocal of my mpg (1 divided by 20.19), I get .049. Multiply that number by 100, and that's 4.9 gallons per one hundred miles (g/100m), which again isn't very great. All right, fine. Slower starts from red lights for me from now on.