As more and more alternative fuel technologies enter the automotive world, the EPA is scrambling to find a way to produce realistic fuel consumption numbers.  All vehicles on sale in the US have a mileage rating, set by the EPA, available.  But cars that are still in development or even concept stages can only estimate what they will get. EPA fuel economy ratings are designed to give prospective owners an idea of relative fuel cost in real-world driving as well as a way of comparing various models on equal ground. The ratings are listed in the EPA’s annual Fuel Economy Guide, at fueleconomy.gov, or on the window sticker of any new vehicle; they’re also used in figuring an automaker’s corporate average fuel economy (CAFE) and in assessing a gas-guzzler tax on some vehicles. But because vehicles like the Volt are still in development stages, they have not yet gotten the EPA’s rating.

In the tests, vehicles are driven under controlled conditions in a laboratory using a dynamometer—described by the EPA as a treadmill for cars. The dynamometer accounts for things like drag and air resistance, and temperature, humidity, and even barometric pressure are all accounted for. Nearly all vehicles are tested by the manufacturers according to strict stipulations, with full test results sent to the EPA; then the EPA confirms between 10 and 15 percent of the results annually at its National Vehicles and Fuel Emission Laboratory in Ann Arbor, Michigan. The two basic test cycles, City and Highway, have been run in much the same way for decades. The City test represents low-speed and stop-and-go driving, with a cold start, a top speed of 56 mph, and an average of just 20 mph, with 23 stops over 11 miles. Maximum acceleration is an extremely gentle 3.3 mph/second though. The Highway test, is intended to replicate free-flow highway driving, with an average speed of 48 mph and a top speed of 60 mph over 10 miles, with no stops and no idling.

Over the years the two sets of test numbers have been adjusted a couple of times to better match the numbers seen in real-world driving. Most recently, in 2008, along with a new fuel economy label for new cars, three new tests were added—High Speed, AC, and Cold Temp—to bring City and Highway estimates more in line with what a high percentages of drivers could obtain in real-world driving.  Regulators decided to use the new five-cycle tests to recognize that there are different effects in different types of vehicles. The new fuel economy tests will have slightly more impact lower-power vehicles and smaller-displacement engines suffering bigger hits because of the AC test, for instance.

From 2008 until 2011, as the new tests are being phased in, manufacturers can choose from performing all five of the new tests to yield revised City and Highway figures, or adjust their figures from existing tests using a set of equations. From the 2011 model year on, manufacturers will be required to run all five test cycles, so don’t be surprised if the numbers change a bit next year, too. Most major automakers have phased in the new method for some but not all of its vehicles this year.