Your mileage may vary--but these days, it appears, your mileage may vary more than it used to.

According to a new report, the gap between rated fuel efficiency and the real-world gas mileage achieved by actual drivers is increasing.

In 2001, it notes, the average difference between rated gas mileage and actual results was 20 percent or less in the U.S. and 10 percent or less in Europe.

By 2012, that gap had risen to 35 percent in the U.S. In Europe, it was 25 percent in 2011.

Highlighted by Ford hybrids

The variation in real-world results against published EPA efficiency ratings has been highlighted by owner reports that the 2013 Ford C-Max and Fusion hybrids do not achieve their 47-mpg combined ratings.

The EPA is now investigating those cars, which appear to have electric motors that, under gentle acceleration, can cover much more of the EPA test cycles in all-electric mode than actual drivers would.

The latest Ford hybrid, the 2013 Lincoln MKZ Hybrid, offers more information to drivers on factors that affect gas mileage.

Most analysis for Europe

The report, From Laboratory To Road, was published this month by the International Council on Clean Transportation.

It looks at ratings and real-world results both in the United States and in the European Union.

The bulk of its analysis applies to European cars and drivers, using data gathered from fleets in Germany, the Netherlands, Switzerland, and the U.K.

It shows the steepest increase in 2007 and 2008, when several member countries switched to taxing cars based on their rated tailpipe emissions of carbon dioxide.

The report is careful to note up front that "nothing in this analysis suggests that manufacturers have done anything illegal."

But, it continues, the New European Driving Cycle test "was not originally designed to measure fuel consumption or CO2 emissions, and some features of the test procedure can be exploited to influence test results for those values."

Discrepancy between EPA gas-mileage ratings & real-world results,

Discrepancy between EPA gas-mileage ratings & real-world results,

U.S. analysis using "My MPG"

Only four pages of the 88-page report apply to similar discrepancies in the U.S.

That analysis is based on variations in "My MPG" data submitted by users to the EPA's website starting in 2004, against the official EPA fuel efficiency ratings.

The report suggests that more study is needed on these discrepancies.

"For the United States," it concludes, "the data examined in the context of this paper are seen only as a starting point for future analysis."


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