The other fuel-efficient technology is a reworked blast from the past: diesel.

When gas prices jumped above $2 a gallon this spring, the price shock got a lot of consumers thinking about how to get better fuel economy.

Someday we may all ride around in cars fueled with hydrogen. But right now two rival technologies are battling for the attention of motorists who value fuel conservation. Gas-electric hybrid vehicles, such as the Toyota Prius, have gotten a lot of buzz. But across the Atlantic, an updated version of old technology has captured a good slice of the market: diesel engines.

In Europe, nearly two decades of intense effort have yielded a new generation of diesel engines for passenger cars that are quiet, smooth-driving and relatively clean. Two German automakers have decided that now is the time to try to convince American consumers to give modern diesels a try.

Recently in the U.S., Volkswagen launched diesel versions of its midsize Passat sedans and wagons. The Passat TDI models (for Turbo-Direct Injection) will join the diesel Jetta, Beetle and Golf TDI compacts in VW's U.S. lineup. VW also is offering 500 of its Touareg sport-utility vehicles equipped with a 310-horsepower, 10-cylinder diesel.

Meanwhile, DaimlerChrysler is getting back into the diesel business in America, offering a diesel-powered version of its new Mercedes E-class sedan. Later this year Chrysler will launch a diesel-powered Jeep Liberty sport-utility vehicle that it says will be 25% more fuel efficient than a comparable gasoline-powered Liberty. That vehicle wasn't available to test-drive at this writing. It will compete with Ford's Escape gas-electric hybrid to define how best to make a fuel-efficient SUV.

The Passat TDI diesel will be an important test of whether the European diesel has a future in the U.S. family-vehicle market. VW wants to sell about 12,000 Passat TDIs a year -- about 10 to 15% of total Passat sales in the U.S. The least-expensive Passat TDI sedan model starts at $23,635, including destination charge, compared with $22,355 for the cheapest gasoline-powered Passat. VW says the diesel is priced at $205 more than a comparable gas model. A better-equipped Passat GLS diesel starts at $25,235, and the diesel Passat wagon starts at $24,635.

VW says the Passat TDI's 2-liter, 134-hp turbocharged diesel engine can get 27 mpg in the city and 38 mpg on the highway, and can cruise up to 623 miles on a full tank. That's the highest EPA mileage rating for any midsize sedan sold in the U.S., according to VW. A Passat with a gasoline-fueled, 1.8-liter four-cylinder engine and automatic transmission is rated at 21 mpg in the city and 30 on the highway.

VW Passat GLS TDI
Base Price: $25,235 including destination charge
MPG: 27 city, 38 highway
Standard: automatic transmission, A/C, power sunroof, side-curtain airbags, 60/40 split-folding rear seat, 15-inch alloy wheels, Monsoon sound system
Comments: Lots of oomph from a standing start but a tad sluggish in highway passing.

The Passat TDI does most things just like a gasoline-fueled Passat. There's no annoying diesel rattle and hum. Steering and cornering are precise, and the car handles abrupt emergency steering gracefully.

The Passat TDI ably handled the uphill and downhill bits of a drive through the hills near Reston, Va. The 2-liter diesel delivers its maximum 247 foot-pounds of torque at just 1,900 rpm, which means it launches away from a stop sign with authority. This feels odd at first, particularly if you normally drive a small-displacement Japanese car. The Passat TDI's tachometer has its redline at 4,700 rpm, which is just when my Subaru WRX is getting going.

In city driving, the TDI diesel's off-the-line oomph is a good thing. Where the Passat TDI and its five-speed automatic transmission are less impressive is in passing maneuvers. Passing a truck on a two-lane road at about 40 mph, the Passat TDI felt sluggish. In freeway passing, the diesel was adequate, not inspiring. Using the manual shift option on the floor-mounted automatic shifter helped, but not much.

VW Touareg V-10 TDI
Base Price: $58,415 including destination charge
MPG: 17 city, 23 highway
Standard: 18-inch alloy wheels, side-curtain airbags, 60/40 split-folding rear seat, power sunroof, intelligent crash response system, 4WD
Comments: A refined, luxurious, powerful beast. Passing is no problem. Obeying the speed limit is.

What would really make a difference is more displacement. That was clear after a fling in the monstrous V-10 Touareg TDI. The 5-liter engine cranks out 310 hp and 553 foot-pounds of torque, enough to pull a 7,716-pound trailer. But the turbo-diesel technology delivers 17 mpg in the city and 23 on the highway. VW doesn't make a gasoline-powered V-10. Probably the most comparable gas engine is the Cadillac Escalade's 345-hp, 6.0-liter V-8, which delivers 380 foot-pounds of torque; it's rated at 13 mpg in the city and 17 on the highway.

Because its engine delivers full torque at 2,000 rpm, the Touareg moves away from stoplights with startling speed for a 5,825-pound vehicle. On the highway, passing is no problem. Obeying the speed limit is.

The Touareg TDI is a beast, but a very refined and expensive beast. It starts at $58,415 but quickly accelerates north of $60,000 with options. The cabin is luxuriously appointed with wood and chrome trim accents that look like wood and chrome, instead of plastic. The Touareg shares a chassis and mechanical components with the pricier Porsche Cayenne SUV. To my eyes, the Touareg is the better-looking of the two.

What's to stop diesels from claiming a big chunk of the market here, as they have in Europe? Clean-air regulations and consumer costs.

The new VW diesels are clean compared with diesels of the past and pump out less CO2 than comparable gasoline engines by virtue of burning less hydrocarbon fuel per mile. But VW's diesels still emit too much fine soot and certain smog-forming pollutants to be sold in California and four northeastern states. VW executives say they are still working to perfect technology to meet tough new clean-air standards that take effect nationwide in 2007.

Meanwhile, consumers will have to reckon whether the savings they'll reap from driving a diesel justify the added purchase costs. If you drove 15,000 highway miles a year, you'd save about $299 annually on fuel by driving the diesel Passat instead of the gasoline Passat, based on fuel prices in late June. Driving the diesel Touareg instead of an Escalade could save you $605 a year.

In Europe, a wider difference in gasoline versus diesel-fuel prices and diesel-friendly tax schemes make diesel economically attractive. In the U.S., buying a eurodiesel puts you on the cutting edge of Continental technology and helps conserve some oil. But it may not fundamentally alter your transportation budget.