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COLUMBIA, Mo. – If you’re in the market for a car that will save you money at the pump, looking at miles per gallon may not be the best way to compare fuel efficiency, says a University of Missouri Extension financial planning specialist.

“People aren’t interested in fuel efficiency to drive more miles. They want to use less gas to drive the same number of miles,” said Andrew Zumwalt, associate extension professor and state specialist for financial planning at MU. “When comparing the efficiency of your old car with a new one, consider using gallons of gasoline used per mile.”

Comparing fuel efficiency in terms of miles per gallon can be misleading, he said. “As the mpg of a car increases, gasoline use decreases, but the savings get smaller with each unit increase.”

For example, if you trade in a car that gets 20 mpg for a car that gets 30 mpg, you’ll save about 167 gallons for every 10,000 miles you drive. If you later trade up to a 40 mpg car, you increase your mpg by the same amount, but this time you only save another 83 gallons per 10,000 miles.

To calculate gallons per 10,000 miles, simply divide 10,000 by the vehicle’s mpg. Your 20 mpg vehicle will use 500 gallons to take you 10,000 miles (10,000 divided by 20 equals 500). At 30 mpg, driving 10,000 miles will use about 333 gallons, reducing fuel consumption by 167 gallons.

A family with two cars could make a costly mistake going by mpg when looking to increase fuel efficiency. Suppose your family has a compact car that gets 34 miles per gallon and a minivan that gets 18 mpg. You’ve decided to trade in one of the cars for a new, more fuel-efficient vehicle.

Are you better off replacing the compact with a model that gets 50 mpg, an increase of 16 mpg? Or should you trade in the minivan for one that gets 28 mpg, an improvement of just 10 mpg?

“Upgrading the minivan would actually save more gasoline even though you would get a relatively small increase in miles per gallon,” Zumwalt said.

Replacing the compact car saves you 94 gallons for every 10,000 miles you drive that car. Replacing the minivan, though yielding a smaller increase in mpg, saves you 199 gallons per 10,000 miles.

Comparing gallons per 10,000 miles makes it easier to see how much you’ll save, Zumwalt said. “If your new car uses a third less gas to get you to work and back, then you’re spending a third less money at the pump.”