Diesel users get shut out of gas plunge party

American motorists have a particularly good reason to celebrate the new year, with the average price of regular, unleaded gasoline ending 2014 near $2.30, according to federal data.

But not everyone is sharing in the good news. According to the most recent figures from the Energy Information Administration, the relatively small number of motorists—along with the nation's truckers—who depend on diesel fuel were still paying an average of $3.21 a gallon as of Dec. 29.

Although that marks a decline of nearly 70 cents compared to the prior year, it's nowhere near the $1.03 plunge in gas prices over the same period.

There are signs that the widening cost gap is starting to short circuit the nascent comeback of the diesel market. At Volkswagen (VOW3-DE), for example—the largest player in the segment—diesel vehicle sales have dipped to less than 20 percent of its overall U.S. sales mix through November. That's down from more than 25 percent earlier in the year.

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"It's too early to call it a trend," but there are clear signs of weakness in diesel sales, said Dave Sullivan, manager of product analysis at AutoPacific.

Sales of diesel-powered vehicles had been on a clear upswing after virtually disappearing in the late 1980s and 1990s. For the first six months of 2014, diesel sales surged a full 25 percent compared to the prior year, while the overall American auto market increased just 4 percent.