Real MPG - Putting the Truth in Your Tank
July 13, 2006

Real MPG - Putting the Truth in Your Tank: A Quarter Century of Deceit

Many drivers know that the mileage number on their car's window sticker is inaccurate, and that cars rarely get the mileage that is advertised. But few people know that the number on the window is not the one that car companies use to meet corporate gas mileage standards. Instead, car company lobbyists and Congress have created what the Center for Auto Safety calls two sets of books when it comes to gas mileage (CAS 2004).

This legally sanctioned deception dates back more than 25 years. In 1980, the U.S. House of Representatives Committee on Government Operations assessed the effectiveness of fuel efficiency standards that Congress had passed in 1975. The Committee found that:

...most new cars sold in this country do not achieve on road the fuel economy standards set by Congress. However, it should be noted that car manufacturers are deemed in compliance with the fuel economy standards mandated by Congress in 1975. American car owners and the entire U.S. economy will both be the losers if that situation is not remedied. Both will pay millions of dollars more, and both will remain more reliant on foreign oil sources, as a direct result of the needless waste of automobile fuel (HCOGO 1980).

The Committee found that model year 1978 cars with government rated fuel efficiency of 27.5 miles per gallon achieved only 19 miles per gallon on the road, a shortfall of 30 percent that is essentially the same shortfall that occurs today. In addition, the Committee found that the shortfall between the government mileage ratings and real world driving could cause the U.S. to burn an extra one million barrels of gasoline per day by 1985.

How the Automakers Drive/How You Drive

The problem, the Committee found, was the grossly inaccurate testing procedures for measuring fuel efficiency; procedures that remain in place today. The city portion of the test is based on a simulated drive to work in Los Angeles in 1965 when congestion was much lower than it is today. The highway portion of the test was developed by the Environmental Protection Agency (EPA) in 1974 and was designed to represent a mix of interstate and freeway driving at a time when the national speed limit was 55 miles per hour. Congress eliminated the national speed limit in 1995 and states have instituted speed limits of 65 or 70 miles per hour (EPA FR 2006).

Not surprisingly, neither the highway nor city test reflects real world driving behavior or gas mileage. One major difference is speed. On the highway test, cars average just 49 miles per hour and never exceed 60 miles per hour. Yet according to the EPA, recent reports show that 28 percent of all driving occurs at speeds greater than 60 miles per hour. Higher speed can dramatically reduce fuel efficiency (EPA FR 2006).

In addition, the acceleration rates in both the city and highway tests are 3.3 miles-per-hour per second despite recent studies of real world driving which show that acceleration rates can be as high as 12 miles-per-hour per second. Rapid acceleration can seriously cut gas mileage. Even when the tests were developed, acceleration rates were higher in the real world. Yet, at the time, the dynamometers (treadmills for cars) on which the tests were completed did not allow for faster acceleration. Today's dynamometers allow for rapid acceleration even though rapid acceleration is not incorporated in mileage testing (EPA FR 2006).

Other significant differences between the driving tests and real world driving are the use of accessories and ambient temperature. Neither the city test nor the highway test uses accessories such as air conditioning, heating or defrosting. Air conditioning in particular can reduce fuel efficiency. In addition, the tests are conducted at 75 degrees F even though only 20 percent of all driving occurs within five degrees of this temperature. Fuel efficiency is lower at temperatures that are above and below 75 degrees.

Finally, because the tests are conducted on a dynamometer rather than on the road, the automakers do not have to account for a variety of conditions that can reduce fuel efficiency including roadway roughness, hills, wind, tire pressure, heavier loads (trailers, cargo, multiple passengers), the effects of ethanol in gasoline and others (EPA Preamble 2006).

A Band-Aid Approach

Despite these discrepancies, Congress and the automakers have resisted real change. In 1980, confronted by inaccuracies in the testing program, automakers General Motors and Ford suggested that the car labels be adjusted so that consumers would have more accurate information. However, the automakers opposed changing the driving test so that they would comply with federal fuel efficiency standards. "That approach is anathema to the auto makers," the Committee on Government Operations reported. Ford argued to the Committee that the testing procedures were fixed by law despite evidence that EPA could have modified the test. The Committee recommended changing the test to accurately reflect mileage for the 1986 and later model years.

In 1984, the EPA adopted the approach favored by the automakers. Since that year, the Agency has adjusted the mileage figures produced by the driving test downward before placing the numbers on cars' window stickers (CAS 2004). The agency lowers the city test figure by 10 percent and the highway figure by 22 percent (EPA 2006). This adjustment provides consumers with somewhat more accurate information. But the test for determining compliance with fuel efficiency standards remains grossly inaccurate.

Congress Ensures that Automakers are Falling Short

The result of the inaccurate driving test is that automakers are failing to meet federal fuel economy standards established by Congress in 1975. Automakers' current fleet of cars must average 27.5 miles per gallon while SUVs and light trucks must average 22.2 miles per gallon. According to EWG's estimate, the true figures are closer to 21.7 miles per gallon for cars and 16.3 miles per gallon for SUVs and light trucks. A recent on-road study by Consumers Union of more than 300 cars and trucks found that 274 delivered lower fuel economy than promised by EPA (CU 2005). Consumers Union found that its estimated mileage for 2003 model year cars was 30 percent lower than that reported to the government for purposes of complying with federal fuel efficiency standards. This discrepancy is the same as that reported by the House Committee on Government Operations for 1978 model year cars.