Comparing Gas Mileage Numbers

September 16, 2016 •

Comparing Gas Mileage Numbers
One factor almost everyone takes into account when shopping for a car is gas mileage. Checking mileage numbers is easy, of course. Just go to www.fueleconomy.gov and look up the vehicles you’re considering, right?

Well actually that’s wrong. Miles per gallon is a misleading way of comparing gas mileage. Yes, it tells you how many miles you can go on a gallon, but what you really need to know is how many gallons you will use to go a certain distance. Let’s use a simple example to illustrate the difference.

Say your current car gets 20 mpg. To drive 1,000 miles, (a typical monthly mileage) you’ll burn through 50 gallons of gas (1,000 divided by 20). Now imagine you buy a new car that gets 25 mpg. That means you’ll only buy 40 gallons of gas (1,000 divided by 25) to go that same 1,000 miles. In other words, you just saved 10 gallons.

Now imagine you’re trading that 25 mpg car for one getting 30. How much gas will you save? Well 1,000 divided by 30 is just over 33 gallons. In other words you’ve saved less than 7 gallons, yet in both examples the new car got an extra 5 mpg.

So here’s the bottom line. If you’re already driving a fuel-efficient car, changing it for one that’s more efficient isn’t going to save you a whole lot. On the other hand, if your truck only gets 15 mpg and you trade it for a sedan getting 25 you’re going to save a ton of money!

So if mpg numbers are misleading, what’s the answer? Well let’s go back to www.fueleconomy.gov. Look up the mileage for any car you like and in small print below the mpg you’ll see a figure for gallons per 100 miles. Using the 2013 Nissan Juke as an example, this gets 29 mpg on the combined cycle and will use 3.4 gallons per hundred miles. Measuring gallons per mile is actually a much better way of comparing fuel efficiency than miles per gallon.

If you’re wondering why we calculate mileage the wrong way, you’re not alone. Experts ask the same. In fact car-buying website Edmunds calls mpg “stupid” and “ill-conceived” while Popular Mechanics says, “… we need to change our way of thinking from high mileage to low consumption.”