How EPA Fuel-Economy Testing Works

How the EPA Tests and Rates Fuel Economy

A Chevrolet Silverado's theoretical fuel economy is evaluated in the EPA's labs.

The EPA doesn't just drive the vehicle to determine how many miles per gallon it gets. Each new car and truck is tested on what's called a dynamometer, which is like a large treadmill. While the engine and transmission drive the wheels, the vehicle never actually moves -- just the rollers upon which the wheels are placed. A professional driver runs the vehicle through two standardized driving schedules, one each to simulate city and highway driving conditions, and ensures he or she is maintaining the mandated pace via a real-time computer display.

The "city" program is designed to replicate an urban rush-hour driving experience in which the vehicle is started with the engine cold and is driven in stop-and-go traffic with frequent idling. The car or truck is driven for 11 miles and makes 23 stops over the course of 31 minutes, with an average speed of 20 mph and a top speed of 56 mph. The "highway" program, on the other hand, is created to emulate rural and interstate freeway driving with a warmed-up engine, making no stops (both of which ensure maximum fuel economy). The vehicle is driven for 10 miles over a period of 12.5 minutes with an average speed of 48 mph and a top speed of 60 mph. Both fuel economy tests are performed with the vehicle's air conditioning and other accessories turned off.

Throughout the test, a hose is connected to the vehicle's tailpipe and collects the engine's exhaust. The amount of carbon present in what's spewed from the exhaust system is measured to calculate the amount of fuel burned. The EPA claims this is more accurate than using a fuel-gauge to physically measure the amount of gasoline being burned. Still, the final fuel economy figures are adjusted downward, by 10 percent for city driving and 22 percent in highway mileage, to help reflect the differences between what happens in a lab and out on an actual road.

How Much Does That Hybrid Get?

As has been well-publicized, the gap between official and experienced fuel economy can be even wider for owners of gas/electric hybrid-powered vehicles. Most experts feel the EPA's ratings for hybrid vehicles tend to be overstated by a factor of at least 20 percent. This discrepancy can be wider yet if a motorist drives primarily on the highway, where hybrids tend to be less efficient than in stop-and-go city driving conditions (during which the electric motor shoulders more of the effort).

Ironically, the results of ongoing operating tests conducted by the EPA of a dozen hybrid cars in its own fleet significantly contradict their posted fuel-economy ratings. According to a report we found on a government website, the best the EPA's fleet could muster was a cumulative average of 37.7 mpg for the Civic, 45.7 mpg for the Insight, and 44.8 mpg for the current-generation Prius. While this is certainly admirable fuel economy, it's still far below the cars' EPA ratings that run as high as 51, 66, and 60 mpg, respectively, for the model years tested.

Why do such discrepancies tend to be more pronounced for hybrids? Experts say it's because basing fuel economy upon the amount of tailpipe exhaust automatically favors gas/electric-powered vehicles. Since some of a hybrid's power comes from an electric motor that automatically produces zero emissions, these figures tend to skew higher than simple miles-driven/gallons-consumed computations would otherwise indicate.

On the next page, we'll look into what driving factors change how many miles per gallon your vehicle gets and why that number differs from what the EPA estimates your vehicle should be averaging.