For longer than four decades, Washington politicians have charged the Environmental Protection Agency with the task of setting fuel-economy standards for motor vehicles sold in the United States. Thanks to the EPA, car shoppers expect to see fuel economy rating labels posted on the windows of new cars in dealer showrooms. The EPA and vehicle manufacturers work hard to ensure that prospective buyers can compare the gas mileage of new cars as part of the buying decision. DAS Demo has an overview of how the EPA gets those numbers.

Testing Labs

EPA regulators use a complex system to arrive at its fuel-economy estimates. The agency tries to develop accurate figures that help consumers know what to expect when they roll out of a showroom in a brand new car. Long before new models and brands of vehicles appear at local dealers, those cars were analyzed to see how many miles they can go on a gallon of fuel. The data used for these ratings come from computerized measurements during simulated driving activity, not actual performance on real roads. EPA laboratory tests rely on special equipment called dynamometers to simulate driving environments. Vehicle wheels rest on dynamometer rollers that spin as drivers accelerate. This way, technicians can conduct stationary driving tests on cars without venturing onto the open road. Engineers can vary the properties of the rollers to simmulate different variables such as the effects of vehicle weight and resistance caused by the wind and road surfaces.

Testing Procedures

Automobile manufactures perform EPA testing on their own products using in-house personnel and equipment. Automakers submit their test results to the EPA for certification before selling new cars. The EPA confirms up to 15 percent of vehicle tests by repeating those tests at its National Vehicles and Fuel Emissions Laboratory. During EPA testing, drivers sit in driver seats and follow a scientifically designed driving regimen meant to simulate typical trips in city and highway driving. Computers connected to the vehicle record performance. Official tests do not rely on fuel gauges to compute miles per gallon. Instead, engineers connect hoses to the tailpipes of cars and analyze the output for carbon content. The amount of carbon tells computers how much fuel the car consumes at any moment.

EPA tests

Fuel efficiency tests put vehicles through several cycles called schedules that test cars with different amounts of idle time, cold and warm engine starts, ambient operating temperatures, maximum speeds, and rates of acceleration. Tests also evaluate cars with the air conditioner on and varying numbers of stops. Every vehicle performs the exact same operations during tests, so auto shoppers have a reliable way to compare the operating costs of the vehicles they consider while shopping.

Exemptions

Vehicle manufacturers do not need to individually test every vehicle or model to comply with EPA regulations. Instead, they test samples based on their weight, engines and transmissions and then apply test data to each combination. This helps streamline the testing process without compromising the accuracy of vehicle ratings. Motorcycles, cargo vehicles that weigh more than 8,500 pounds, and passenger vehicles that weigh more than 10,000 pounds do not require fuel efficiency testing under EPA rules. This means some large consumer-oriented pickup trucks, SUVs, and vans may not require EPA assessments.

Reliability

Drivers operate their cars on real streets and highways, not in testing laboratories, so vehicles often perform better or worse than indicated on EPA labels. Actual fuel performance of vehicles varies based on factors such as road conditions, operating temperatures, driver behavior, vehicle condition, and fuel. Aware of this, auto shoppers should use the EPA estimates as shopping tools rather than guarantees of performance.