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Personal Finance > Autos

'Your mileage may vary'
The EPA's fuel efficiency estimates are in the hot seat these days
October 8, 2004: 7:40 AM EDT

NEW YORK (CNN/Money) - There's been a lot of griping lately about the Environmental Protection Agency's fuel efficiency estimates. Those estimates are pretty far off, they say.

The discrepancies are becoming a big deal because cars are getting more fuel-efficient making differences more noticeable. For example, by some estimates EPA mileage ratings tend to overstate fuel efficiency by about 15 percent.

If that's true, then drivers of GMC Yukons are getting about 13 miles to the gallon instead of the 15 the EPA estimates. That's so close that, as an estimate, it may as well be dead on.

Buyers of Toyota Priuses, however, would have something to complain about. The EPA estimates that it should get about 55 miles to the gallon. If the EPA's figure is off by 15 percent that would mean a real-world fuel economy of about 46 miles per gallon. That's a noticeable difference, especially since the Prius has a computer screen that continually informs drivers of its fuel efficiency. It also roughly conforms to many real-world reports including CNN/Money's own tests with the car.

Lab tests

The EPA's fuel efficiency tests do not involve driving the cars nor do they involve actually measuring the flow of fuel. Instead they are based on laboratory tests. The good thing about lab tests, the EPA says, is that they are highly consistent from one test to the next and one vehicle to the next.

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That means that test results can reliably be compared to one another. It may not mean that the results can be reliably compared to what happens when you drive a car on real roads.

The tests are done on a machine that allows the car's wheels to spin without the car actually moving anywhere. Instead of measuring the fuel flowing into the engine, the EPA measures the amount of carbon leaving the tailpipe. Since carbon isn't lost in combustion, the amount of carbon in the exhaust fumes indicates the amount of fuel going into the engine.

The "city" driving test is designed to mimic an 11 mile stop-and-go trip with an average speed of about 20 miles per hour. About 18 percent of the time is spent idling and the test includes a short "freeway driving" segment.

The "highway" test is designed to mimic a 10 mile trip on a two-lane country road, not an interstate, said John Millett, an EPA spokesman. The average speed, including slow-downs to mimic allowing another car to merge into traffic, is about 48 miles per hour.

Since these tests were created, driving habits have changed. Adjustment factors were added into the equation in 1985 as a simple way to make things right again. After all the testing is done, the EPA reduces the city mileage by 10 percent and the highway by 22 percent.

Real world driving

But still, consumers say, they're not getting the mileage the EPA says they should be.

One reason probably has to do with driving speeds, said Millett. The EPA's highway mileage rating system was never designed to mimic interstate driving, he said. Today, people are comparing those numbers to mileage they're getting on interstates where speed limits are as high as 75 miles per hour. Higher speeds mean lower gas mileage.

Accessories are another issue. Air conditioning was not nearly as common in cars in the 1970's as it is today. Today air conditioning is common in cars and drivers use it often, especially in warmer months, without even thinking about. The air conditioner is left turned on and it just starts when the car starts. In an average mid-sized car, the air conditioner uses about three or four horsepower, according to information provided by General Motors.

Air conditioning actually puts a burden on the engine. That's no big deal for a powerful V-8 engine, but for the sort of small engine that powers an economy car or, even worse, a gas/electric hybrid, air conditioning can be big drain on fuel efficiency.

The EPA is currently reviewing plans to change its test procedures to reflect more modern driving habits. In the end, they may decide no change is needed.

In the meantime, remember that old line.... "Your mileage may vary."

In fact, it almost certainly will.  Top of page

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