Are These Cars "Greener" Than Hybrids?

You hear the stories from just about any American who’s driven in Europe—wide-eyed descriptions from the fuel-economy frontier. Rambling along the Riviera in midsize sedans that get 40 or more miles to the gallon. Streaking along the Autobahn in high-performance luxury cars with the more demure fuel-consumption habits of a Honda. And doing it all in, not a hybrid or even a car that uses regular gas—but in a diesel. For anyone who still thinks of diesel vehicles as sluggish, sulfur-spewing machines that clatter their way up every hill, here’s a news flash: These aren’t your daddy’s disco-era diesels. Unlike the cars popularized during the 1970s energy crisis (when diesel fuel was cheaper than regular gas), today’s diesel-powered vehicles run quietly, accelerate quickly and meet emissions standards in all 50 states—even smog-conscious California. According to the U.S. Department of Energy, today’s so-called clean diesel engines are not only more powerful, but they’re also 30 to 35 percent more fuel-efficient than comparably sized gas ones.

And they’re starting to pop up on this side of the pond. Over the next five years, industry analysts expect the share of diesel vehicles sold in the U.S. to more than triple its current 2.2 percent. (Hybrids’ market share also hovers around 2 percent.) Not surprisingly, most of the new models hail from European makers who’ve begun to offer more fuel-efficient diesel versions of popular luxury and performance models like the Audi Q7, BMW 335 sedan and several Mercedes SUVs. Volkswagen, meanwhile, is unveiling an affordable, high-mileage vehicle poised to give eco-darling Toyota Prius some serious competition. Indeed, a panel of automotive and environmental experts recently named Volkswagen’s Jetta TDI, with an EPA highway rating of 40 mpg and a starting price of $22,550, the “green car of the year”— the first diesel to earn that honor.

Could the new diesels dethrone hybrids? We recently test-drove a Mercedes ML320, a midsize diesel SUV, and experienced firsthand both the joys and frustrations of diesel technology.

Driving a diesel these days defies expectations at every turn. Having piloted our share of mushy-pedaled hybrids, we don’t exactly equate fuel sipping with spirited performance. But in the ML320, we bound from stoplights like a 4,800-pound jackrabbit, since the diesel engine delivers some 50 percent more torque, or pulling power, than its gas equivalent. And while we’d come to expect rattles and rumbles from the diesels of yore, our six-cylinder Mercedes is so sound-insulated, we feel like we’re enveloped in Maxwell Smart’s “cone of silence.”

All the while, our fuel gauge inches slowly down. How slowly? Like with gas and hybrid vehicles, it depends largely on vehicle and engine size. With SUVs and sportier models (a natural fit for brawny diesel engines), the savings over regular-gas models run generally between 3 and 5 mpg—nothing that’ll wow the green brigade but comparable with hybrid performance for these types of cars. The Mercedes ML320 averaged mid-20s for us, compared with the 17 combined city/highway rating for its gas counterpart. But going smaller can yield dramatic results: A European Civic diesel we drove (not available in the U.S.) averaged 40-plus mpg—10 more than its gas counterpart and up there with top-performing hybrids.

Of course, mileage also depends on driving style. Diesel engines maximize their efficiency on the open road, where the hybrid’s battery pack is useless dead weight. Hybrids do better in stop-and-start driving, since their combustion engines automatically shut off while idling. And avoiding lead-foot syndrome can make a real difference; while the VW Jetta is officially rated at 29 mpg city and 40 highway, reports of 50-plus have surfaced. One hypermiling couple recently landed in the Guinness Book of World Records by earning over 58 mpg on a road trip in the new diesel Jetta.

Still, on the pocketbook front, the news could be better. Because most of these new diesels come from European luxury makers (Detroit’s diesels are mostly commercial-grade and light-duty trucks), not too many qualify as budget-minded. And as with hybrids, you’ll pay an average premium of almost $5,000 for your low-CO2 bragging rights. (One exception: Mercedes has kept the difference between its gas and diesel M-Class models to about $1,500.) Also, diesel fuel currently costs 32 percent more per gallon than regular gas, erasing much of the potential savings at the pump—a fact that has caused some makers, like Honda, to consider delaying plans for a U.S. diesel rollout. According to the Environmental Protection Agency, buying a diesel M-Class could run about $130 more a year in fuel costs than the gas model, and about $700 more than a comparable Lexus hybrid.

There is some good news. The U.S. government is dangling some incentives for clean diesels: $1,300 for the Jetta TDI, $900 for the Mercedes ML. But until tax breaks broaden, diesel fuel costs improve and manufacturers put diesel engines into more small cars, these vehicles will likely remain a niche for Americans who appreciate potent performance but want a touch of green.