Beware The "MPG Illusion" When Comparing Fuel Efficiency

Sure, switching from a gas guzzler to a highly efficient (and probably much smaller) car is best for the environment, but it’s not a realistic solution for large families or people who can’t afford it. But don’t let the fact that you can’t buy a 40 mpg car turn you off of a trade up in efficiency anyway. A couple of economists have pointed out that “using ‘miles per gallon’ as a measure of fuel efficiency leads people to undervalue the benefits of replacing the most inefficient automobiles.” Their point: if you’re driving a gas guzzler, even a small improvement in fuel efficiency can generate significant savings.

Gillis calculated that at $4 a gallon, over 10,000 miles, an improvement from 12 mpg to 13 mpg would save $256. For the owner of a 33 mpg car to save that much, mileage would have to go up to 40 mpg, he said.

Here’s how it works.

A couple drives a 25 mpg sedan. They trade it for a 50 mpg hybrid, a 25 mpg improvement.

A family with mom, dad and three kids has a 10 mpg SUV to haul everyone around. They trade it for a 20 mpg station wagon, a 10 mpg improvement.

Sounds like the couple did better, at least in miles per gallon.

But lets look at gallons per miles.

At 25 mpg the couple burned 400 gallons over a year and their new 50 mpg hybrid cuts that to 200 gallons. They save 200 gallons.

At 10 mpg the family’s SUV burns 1,000 gallons of gas a year. At 20 mpg the station wagon burns 500 gallons — they save 500 gallons, much better than the couple.

Obviously you stand to save the most with the most efficient car. In the above example, though, you’re spending so much on gas for that big vehicle that cutting your consumption in half can save you $2,000 a year. It’s worth keeping this in mind if you’ve been assuming it’s cheaper to stick with your old 10 mpg vehicle, or if you’re car shopping on a limited budget and tempted to disregard modest fuel efficiency ratings.

“Seeking better gas mileage? Think backwards” [CNN]
“The MPG Illusion” (subscribers only) [Science]
(Photo: Getty)