Why You're Not Getting the Mileage Printed on the Window Sticker

BYJim Prueter
When it comes to the miles per gallon printed on a new vehicle’s window sticker, what you see isn’t necessarily what you get. It’s more like a rough estimate. There are a variety of reasons few drivers actually achieve those mileage numbers, including the U.S. Environmental Protection Agency’s testing methodology, the vehicle itself, and the driver’s habits.
How Fuel-Economy Testing Works
So how does the EPA arrive at the estimates you see on the car’s window sticker? Contrary to popular belief, the testing is not done by the EPA. The testing is done by the automakers, who use standardized testing protocols developed and established by the agency. The EPA then spot checks 10 to 15 percent of the models at the National Vehicles and Fuel Emissions Laboratory in Ann Arbor, Michigan, to confirm the results sent to them by the automakers.
Fuel-economy testing first began back in 1975 using dynamometers. Back then, highway speed limits were mandated at 55 mph and most vehicles used regular gasoline. The dynamometers used for mpg testing in the labs are incapable of replicating the performance of today’s vehicles. The simulations assumed less congested highways and presumed that drivers spent more than half their driving time on city roads. Most commuters today, however, are racking up more time on the highway.
In 2008, the EPA modified the testing to accommodate and anticipate the latest technology on today’s cars and trucks, like regenerative braking on hybrid vehicles, stop-start engines, the growing switch to electric air conditioning, electric power steering, higher highway speeds, the effect of hot and cold weather and air conditioning, and other factors that cause gaps between lab results and real-world driving. Those changes brought the numbers a bit closer, but meaningful differences still remain.
Reasons for the Mileage Gap
If there is one glaring problem with the EPA’s testing methods, it’s that they require pure petroleum gasoline. The majority of fuel being pumped into vehicles today is blended gasoline, diluted with 10 percent (or more) ethanol. These blends reduce the vehicle’s mileage by 4 to 5 percent.
Mileage also is affected by the condition and use of the vehicle itself, such as completion of scheduled maintenance; dirty spark plugs and air filters; underinflated or poorly aligned tires; extra occupants; and cargo. Roof, bike, and cargo racks only make matters worse. Outside temperature and terrain, such as mountains or twisty roads, also make a significant difference.
Driving habits have the largest impact on fuel efficiency. Driving fast, increased stop-and-go driving, excessive idling, and frequent acceleration and lane changes can result in large reductions in miles per gallon.
Our recommendation? Use the mpg fuel economy notation on the window sticker as a general guide, not gospel.
JIM PRUETER, an automotive writer based in Phoenix, has provided reviews and advice about cars for more than 20 years.
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