As gas prices dropped and sales of less-efficient vehicles increased, improvements in U.S. new-car average fuel economy came to a halt in 2015.

Two separate analyses show that the average fuel economy of new cars sold in the U.S. slowed in 2014, and decreased last year.

The sales-weighted average window sticker fuel economy of new cars for calendar year 2015 was 25.3 mpg, according to researchers Michael Sivak and Brandon Schoettle of the University of Michigan Transportation Research Institute (UMTRI).

DON'T MISS: Will Cheap Gas, Soaring SUV Sales Sink Fuel-Economy Goals? (Dec 2015)

That's down 0.1 mpg from the value recorded for 2014.

Average fuel economy declined through the end of the year, with December's average registered at 24.9 mpg--down 0.2 mpg from November.

It also represents a 0.9 mpg drop from the peak reached in August 2014, although it is still 4.8 mpg above the value for October 2007, when UMTRI began monitoring fuel economy.

Gas pump

Gas pump

Researchers attribute the fuel-economy decreases to low gas prices and increased sales of SUVs and light trucks.

Meanwhile, the U.S. Environmental Protection Agency (EPA) recently released its own fuel-economy figures for 2014, showing a small overall increase during that year.

Average fuel economy for new cars rose 0.4 mpg compared to 2013 to 24.3 mpg, the agency said.

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That's a much smaller gain than the 1.2 mpg recorded by the EPA between 2011 and 2012, according to the Associated Press.

The culprit again appears to be SUV sales, which have been on the rise since 2013 as consumers take advantage of lower gas prices.

SUVs and trucks accounted for about half of new vehicles sold in the U.S. in 2013, with 7.82 million sales, according to Autodata Corp.

2016 Ford Explorer

2016 Ford Explorer

Sales rose to 8.6 million in 2014, and reached 8.74 million in the first 11 months of 2015.

But regardless of the current slump, new-car fuel economy will have to improve.

MORE: Miles Driven In U.S. Hits New All-Time High; Gasoline Used Doesn't (Nov 2015)

Impending Corporate Average Fuel Economy (CAFE) standards require automakers to achieve a fleet average of 54.5 mpg by 2025.

That's equivalent to about 40 mpg on the window sticker.


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