That increase was due, in part, to several automakers boosting their diesel fleets over the past several years.
Audi, for example, brought out three new "oil-burners" for 2014, and another for the current model year. In 2013, Chevrolet launched its first diesel-powered passenger car in decades, a version of the Chevrolet Cruze. Also for the 2014 model year, Chrysler added the Ram EcoDiesel pickup, winning over new buyers and a number of prestigious awards.
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Though diesel fuel has traditionally been more expensive, it's usually only about 25 cents or 50 cents pricier, depending on the state. And because vehicles that run on diesel are more fuel efficient than traditional gasoline-powered cars, drivers were often willing to shell out the extra dough.
Now, however, there are a number of factors driving the larger gap between gas and diesel prices.
Local tax policies are one culprit, as some states charge more for diesel fuel to make up for higher road maintenance costs caused by heavy truck traffic. Another factor is that the U.S. now exports a significant portion of the diesel fuel it produces, causing domestic prices to rise from the competition.
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And because diesel fuel is also used to heat homes, a severe cold snap can also push prices higher, Sullivan said.
Still, it remains to be seen whether the gap between gas and diesel fuel will continue to widen, and if so, for how long. As for whether sales of these vehicles will continue their downward trend, December's auto sales results—to be released Monday—should offer a clue.