Measured under controlled circumstances in a laboratory, fuel economy of a vehicle is measured using a standardized test procedure that is specified by federal laws. Carmakers test their own vehicles and then report the results to the EPA (Environmental Protection Agency).
In the laboratory, the vehicles drive wheels are placed in a dynamometer, a machine that is like a large treadmill. The dynamometer stimulates the driving environment. The rollers of the dynamometer can be adjusted to account for the weight of the vehicle and to mimic wind resistance.
While the vehicle is on the dynamometer, a professional driver puts the automobile through a standardized schedule that simulates a typical driving trip in the city and then another test that will simulate highway driving.
Each test dictates the speed in which the vehicle must travel during each moment of the test. The driver will watch a computerized display, which will show his or her driving statistics to ensure they are following the specific schedule.
- The city driving program: Designed to simulate a rush-hour experience. The test is done when the engine is cold and includes frequent idling and stop-and-go traffic. The automobile is driven for a total of 11 miles with 23 stops. This test lasts for a total of 31 minutes. The speeds for this test average 20 miles per hour with a top speed of 56 miles per hour.
- The highway-driving program: Designed to simulate freeway driving with a warm engine. There are no stops included in this test. The automobile is driven for a total of 10 miles for 12.5 minutes and includes an average speed of 48 miles per hour with a top speed of 60. All accessories, such as the air conditioning and heat are off during both of these tests.
In automobiles that use gas, natural gas or diesel, a hose is attached to the exhaust to capture all of the engine exhaust during the test. The amount of carbon present in the exhaust is calculated to determine the amount of gas that is burned during the test. This result is more precise than using a fuel gauge.
Environmental Protection Agency Ratings vs. Actual Fuel Economy
The automobiles that are used for testing are new vehicles that have been broken in and are in perfect mechanical condition. When actually driving a vehicle, several factors can influence the actual fuel economy. For instance, minor upkeep factors such as incorrect air pressure in the tires can affect fuel economy.
Testing is also done without any cargo, passengers or accessories such as air conditioning engaged. All of these things play a role in the actual mileage. The heavier the automobile is, the more fuel it will burn to reach and maintain its speed. Other factors that can affect your actual mileage include weather, terrain, traffic conditions and trip length.
A Useful System
Even with the inaccuracies of the EPA fuel economy vs. actual fuel economy, this information is useful. By using this information, new car shoppers can get a rough idea of how a particular automobile will perform against others in its class. If you are comparing two vehicles and one is estimated to get better gas mileage, you can safely assume that you will be paying more to keep one gas tank full over the other. This rating is also valid for identifying the relative efficiency of an automobiles model range.