The Unexpected Way Car Manufacturers Calculate Gas Mileage

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While you were out shopping for your current vehicle, we’re sure you may have glanced at the manufacturer’s sticker with your car’ expected mpg, but did you take a moment to think about just how do they calculate vehicle mpg?

Automobile manufacturers test vehicles in laboratories, under controlled conditions using standardized testing procedures provided by the federal government. And it’s often pre-production prototype vehicles that are tested. It’s a bit unexpected, isn’t it?

After the manufacturer completes testing, results are sent to the Environmental Protection Agency (EPA), who is responsible for reviewing and confirming results using their own testing procedures. It’s common for the EPA to only confirm 10% to 15% of manufacturers testing results.

In order to test each vehicle, the manufacturer uses a dynamometer to simulate city and highway driving environments, using rollers to account for wind resistance and vehicle weight. The test is performed by professional drivers who are responsible to follow the provided driving schedule, which alerts the driver to how fast or slow he or she should be travelling at that given second.

A hose is connected to the tailpipe of cars fueled by carbon-based fuels, i.e. gasoline, diesel, and natural gas. The hose is then used to collect engine exhaust throughout the test, which is measured to determine how much fuel was burned during the test. Carbon exhaust measurements are more accurate than a vehicle’ fuel gauge.

Watch the video from the U.S. Department of Energy to see what happens during a test. Now there is only one question left to ask, just how did you think vehicle mpg was calculated?