How Do Automakers Calculate Fuel Economy and Range?

Self-testing using EPA drive cycles is the basis of window sticker MPGs.

Benjamin Hunting | 

Aug 3, 2022 | 3 min read

Ford

All cars, trucks and SUVs sold in America undergo extensive fuel economy testing before they hit the dealership. Electric vehicles (EVs) undergo a similar process to determine their driving range. What you might not know is that those tests aren't performed by a government agency, but by the automakers themselves in facilities of their own choosing. There are simply too many vehicles on sale for any federal agency to independently test each one.

While the overall efficiency certification program is overseen by the Environmental Protection Agency (EPA), car companies are given a fair amount of latitude when self-reporting on individual vehicle mileage and range ratings. Here's how the overall process works.

Self-Testing Is Standard Practice

Car companies handle the nuts and bolts of fuel economy testing in the United States, but the tests themselves are defined by the EPA. Performed in a lab with a dynamometer attached to a vehicle's drive wheels, the tests run through “drive cycles” that simulate city and highway driving, including high-speed and stop-and-go environments. Gas- and diesel-powered vehicles pass through five specific tests, while electric vehicles undergo something called the Multi-Cycle City/Highway Test Procedure to reflect the variety of conditions U.S. drivers might face.

Not All Models Are Tested

Many vehicles are offered in a variety of configurations, some of which share the same drivetrain. In these cases, not every trim level is required to go through the same regimen of fuel economy testing. The EPA allows manufacturers to run only partial tests or no tests at all for certain model variations, depending on the automaker’s sales volume projections and what those vehicles weigh. This means vehicle options such as more aggressive gearing or larger wheels could drop real-world fuel economy below the number being advertised.

Cheaters Do Get Caught

The EPA does make sure to test 15% to 20% of new automobiles at its own labs to confirm that companies are providing customers with accurate numbers regarding fuel economy.

Still, that is a small percentage. There have been many instances of automakers either making mistakes during the testing process that resulted in inaccurate fuel economy figures being published, as with Hyundai and Kia in the early 2010s, or deliberately cheating during the testing process, as Volkswagen did with its diesel vehicles.

The EPA also has a system in place to respond to owner complaints regarding fuel mileage problems. This is how an issue with Ford's C-Max hybrid fuel economy was discovered. The C-Max, a small, van-like vehicle, wasn't tested for fuel economy by Ford. Instead, the company used the rating from its Fusion hybrid sedan, which fell into the same weight class and used an identical drivetrain. This was legal under testing guidelines at the time, but the C-Max’s taller shape led to aerodynamics that dragged down its fuel efficiency. After consumers complained, Ford relabeled the C-Max to match the EPA’s estimates and paid out $19 million as part of a lawsuit related to false advertising.

Benjamin Hunting

Benjamin Hunting is a writer and podcast host who contributes to a number of newspapers, automotive magazines, and online publications. More than a decade into his career, he enjoys keeping the shiny side up during track days and always has one too many classic vehicle projects partially disassembled in his garage at any given time. Remember, if it's not leaking, it's probably empty.