New-car buyers place a lot of faith in the EPA's fuel economy numbers on the window sticker -- only to find out later that the vehicle doesn't meet them.
That's the conflict behind the standard industry disclaimer, "Your mileage may vary."
For various reasons, most new vehicles aren't going to get the EPA fuel economy numbers indicated on the window label, no matter how carefully they are driven.
The EPA's tests can't take into account such factors as temperature and geographical differences, fuel blends and quality or individual driving styles.
"Everybody wants a label that tells you exactly what you're going to get, but obviously that's not possible," Jeff Alson, a senior EPA engineer, says. "A good general rule of thumb is that real-world fuel economy is about 20 percent lower than the lab numbers."
But the EPA's own testing methods also contribute to the gap. Here are some of the reasons:
- Fuel: The gasoline used by the EPA to test vehicles in its fuel economy lab contains no ethanol. Gasoline at most of the nation's fuel pumps contains as much as 10 percent ethanol, and that can lower fuel economy by about 4 percent, according to the EPA.
- Sample size: There are more than 260 nameplates from all automakers available in 2013. But the EPA tests just 10 to 15 percent of them. The agency relies on automakers to test their own vehicles and submit data for review. Also, automakers usually bundle one set of EPA city and highway figures for all the variants of a particular nameplate.
- Driving conditions: EPA testing does not mirror real-world driving. For example, the EPA's highway test procedure is calculated using a top speed of 60 mph and an average speed of just 48.3 mph in "free-flowing traffic." City driving tests assume an average speed of 21.2 mph.