It wasn't so long ago that the superior fuel efficiency of diesel-powered vehicles could be rationalized away due to the higher per-gallon price of the fuel compared to gasoline. Now, however, a dip in the price of diesel has pushed it below gas, and lit sales on fire in the process.

The price difference is significant, especially when taken in light of the stronger fuel efficiency of most diesel engines. According to the Department of Energy, the average price for a gallon diesel as of June 22 is $2.61, while the average price for a gallon of regular unleaded gasoline is $2.69. If your car requires premium, that'll be another $0.25 or so - but you already knew that.

So now that it's not just more efficient, but also cheaper, to drive a diesel, it's no surprise that Americans are flocking to the few carmakers that offer them. In fact, Volkswagen saw a rise of 71% in sales of the Jetta TDI between April and May, reports AutoWeek. They are expecting to do just as well or better in June.

Mercedes-Benz is also benefiting hugely from the sales trend, in part because it offers both the M-Class and GL-Class SUVs with diesel powertrains, in addition to the R-Class crossover. Between the three, Mercedes has sold about 15,000 diesels so far this year in the U.S.

BMW could also see some benefit from the lower fuel prices thanks to its offering of the 335d and X5 xDrive 35d. Those vehicles, along with the Volkswagen and Mercedes models, are also eligible for a tax break of between $900 and $1,800 under a provision of the Lean Burn bill.