For drivers whose travel include long stretches of high-speed highways, diesel cars offer a fuel-efficient alternative to gasoline vehicles.
For many years, the biggest seller of diesel vehicles in the U.S. has been Volkswagen--more recently joined by its corporate siblings Audi and Porsche.
MORE: Volkswagen Jetta TDI: Much More Mileage Than EPA Admits?
Sales of an expanding range of diesel models from all three brands grew last year, pointing out one way the trio will meet increasingly tough corporate average fuel economy standards in years to come.
Last year, the Volkswagen brand sold 95,823 diesel passenger vehicles in the U.S., an increase of 6.1 percent over its 2012 total of 90,294. Its TDI diesels represented 23.5 percent of the 407,704 VWs sold in the country last year.
2014 Audi Q5 TDI, Catskill Mountains, Oct 2013
Audi sold 10,076--up exactly 40 percent on 7,195 in 2012--and Porsche added 5,386 more, a huge jump of 333 percent over the 2012 total of 1,618.
Adding the three makes together, that's a VW Group total of 111, 285 diesel vehicles last year--up 15.8 percent over the 96,107 sold in 2012.
Highest volume: Passat
The main diesel model for Volkswagen is the mid-size Passat TDI sedan; almost one-third of the 109,652 Passats sold last year were diesels, or 34,963.
That's followed by the Jetta TDI (26,587, or 18.8 percent of 141,589 Jettas) and the Jetta SportWagen TDI (17,396, or fully 77.2 percent of 22,534 SportWagens sold).
Other diesel models included the Golf TDI (almost 30 percent of the 25,332 Golfs delivered), the Touareg TDI (almost 45 percent of the 8,233 sport utilities delivered), and both the Beetle TDI (11.8 percent) and Beetle Convertible TDI (14.8 percent), which totaled more than 5,600 together.
Adding Audi, Porsche
Audi's TDI diesels include the A3 TDI compact hatchback (about to be replaced with an all-new model) and the low-volume Q7 TDI large luxury sport-utility vehicle.
For 2014, Audi has added TDI diesels to four more models, including its A6 and A8 sedans, the A7 hatchback, and the Q5 compact crossover utilty.
MORE: 2014 Audi Q5 TDI Diesel Crossover: Fuel Economy Test
Porsche offers a model of its Cayenne sport-utility vehicle with a diesel as well, accounting for its contribution to the total.
2014 Volkswagen Passat TDI
Diesels vs electric cars
For perspective, the VW Group total is higher than the total sales of more than a dozen plug-in electric vehicles sold last year in the U.S. That number was roughly 96,000.
By comparison, Volkswagen also sold 5,655 hybrid models, all but 118 of them its new-for-2014 Jetta Hybrid sedan (the rest were Touareg large sport utilities).
Audi also has a very low-volume Q5 Hybrid, and Porsche offers both a Cayenne S Hybrid sport utility and a hybrid Panamera S sedan that's about to be replaced with a plug-in hybrid model of the same car.
It will be interesting to see whether the overall growth rate of plug-ins this year is matched by the growth rate of diesels, as the number of vehicles offered with diesels has jumped significantly for 2014.
Those include two high-volume U.S. models--the Chevy Cruze and the Jeep Grand Cherokee--as well as the Ram 1500 EcoDiesel, the first light-duty full-size pickup truck offered with a diesel in many years.
It all goes to point out that car buyers will have an expanded universe of greener vehicles from which to select in the years to come.