Most new cars and light trucks sold in the United States bear window stickers that list city and highway fuel economy ratings—an indispensable resource for consumers shopping for a fuel-efficient vehicle. Unfortunately, many drivers will find that their cars don't deliver the gas mileage these ratings suggest. So how do automakers arrive at these numbers?
Mandatory fuel economy tests for new vehicles follow procedures defined by the Environmental Protection Agency (EPA). The two tests currently in use, however, are more than 30 years old and do not represent current driving trends (see sidebar). The oldest, the Federal Test Procedure (FTP), is adapted from a test in which six EPA staffers took turns driving a 1969 Chevrolet around Los Angeles. The Highway Fuel Economy Test (HFET) was created soon afterward and has gone unchanged ever since.
Stuck in the Past
Both tests involve placing a vehicle on a dynamometer, a treadmill-like machine that allows the car's wheels to rotate while the vehicle itself remains motionless. A professional driver operates the vehicle at pre-defined speeds and acceleration rates over a specified timetable. The FTP, for example, attempts to simulate city driving conditions using an average driving speed of 21 miles per hour (mph) and a maximum speed of 57 mph. To simulate highway conditions, the HFET uses a higher average speed of 48 mph and a maximum speed of 60 mph. Automakers report the results to the EPA, which then repeats the tests on about 10 percent of vehicle models to verify that the reported results are consistent with its requirements.
 |
| A vehicle's fuel economy is measured by placing it on a dynamometer and running it at government-specified speeds and acceleration rates. |
Not long after the EPA created these tests, consumers began complaining that the real-world fuel economy of their cars did not measure up to the published test results. But instead of revising the tests to yield more accurate results, the EPA developed adjustment factors in the 1980s to lower the test results. Thus, today's window stickers reflect FTP test results that have been reduced by 10 percent (city driving) and HFET test results that have been reduced by 22 percent (highway driving).
While fuel economy ratings were adjusted to provide consumers with more accurate information, the adjusted ratings are not used to determine automakers' compliance with federal fuel economy standards. This is because of a provision in federal law that stipulates an automaker's compliance with Corporate Average Fuel Economy (CAFE) standards must be based on the same tests used in 1975.
|
Life in the Fast(er) Lane
The calculations behind federal fuel economy ratings have not changed since 1985. That neglects a number of recent trends affecting vehicle performance.
Higher speeds. Fuel economy testing is conducted at speeds below 60 mph despite the fact that most states now have both rural and urban interstate speed limits of 60 mph or higher. Fuel economy drops about 10 percent between 55 mph and 65 mph, and 17 percent between 55 mph and 70 mph.
Faster acceleration. Fuel economy testing is based on maximum acceleration and deceleration rates of 3.3 mph per second (roughly comparable to going from 0 to 60 mph in 18 seconds), but the average new vehicle can reach 60 mph in less than 10 seconds. This extra power burns more fuel.
Increasingly congested roadways. The amount of time drivers waste in traffic jams nearly quadrupled between 1982 and 2001, and EPA data show that nearly two-thirds of driving is done in urban areas. Idling drastically reduces fuel economy.
Shorter trips. EPA data from the early 1990s show that the average trip has shrunk to less than five miles, but fuel economy testing assumes a trip length of 11 milesbroken up by a 10-minute rest. Shorter trips increase the percentage of miles driven while a car is still warming up and operating below peak efficiency.
Heavier engine loads. Accessories such as air conditioners and defrosters place a significant load on a vehicle's engine. Though air conditioning is now standard on 99 percent of new cars, fuel economy tests are conducted without the air conditioner running, and only some manufacturers run the defroster.
New technologies. Cars and trucks with gasoline-electric hybrid drivetrains, cylinder deactivation (which shuts off half of an engine's cylinders when they're not needed), and other advanced technologies may react differently to changes in driving patterns compared with conventional vehicles.
| A Welcome Change
With driving habits and vehicle technologies continuing to evolve the current EPA fuel economy tests are sorely in need of revision. These tests are conducted at room temperature, which fails to reflect the extremes encountered during real-world driving. Moreover, the agency's adjustment factors are so outdated that drivers may be getting 10 percent fewer miles per gallon on average than the window sticker suggests. For certain types of vehicles and certain driving conditions, the disparity may be even greater. The best way for the EPA to ensure accurate fuel economy ratings would be to conduct a comprehensive study of real-world driving patterns and fuel economy. The new test cycles resulting from this study should account for current driving habits, accessory usage, and environmental conditions.
On January 10, 2006, the EPA released a Notice of Proposed Rulemaking that represents a positive step toward this ultimate goal. The agency's proposal would redefine the distinction between city and highway driving, and introduce additional test cycles including:
- US06, which simulates high-speed/aggressive driving, reaching a maximum speed of 80 mph and a peak acceleration of 8.4 mph per second
- SC03, which simulates driving in hot weather (95 degrees Fahrenheit) combined with air conditioner use
- Cold FTP, which simulates driving in cold weather (20 degrees Fahrenheit)
These tests are currently conducted on a limited number of vehicle models to measure smog-forming and other emissions. To assess fuel economy, the EPA will combine the results with those from FTP and HFET tests. The agency expects these new numbers will better represent current driving patterns.
The EPA's proposal is a step in the right direction, as it acknowledges the importance of providing better fuel economy information to consumers who want to make environmentally and financially sound decisions. It also leaves the door open to incorporating new data as they become available, which is critical to keeping government ratings as accurate as possible.
Don MacKenzie is an engineer in the Clean Vehicles Program.
|
