Size/Market Segment: Tougher Tier 2 federal emissions standards have curbed the sale of diesel passenger cars in the U.S. over the last couple of years. Most manufacturers are still working on new emissions control technology to meet the current requirements. That said, diesel engines have remained consistently available in three-quarter-ton and one-ton pickup trucks and full-size vans.

If you want a diesel in a light-duty 2008 vehicle, you'll be limited to the Jeep, Mercedes-Benz and Volkswagen brands. Mercedes has the widest coverage, offering a turbodiesel V6 in sedan, wagon and SUV body styles. Jeep offers a diesel solely on the Grand Cherokee midsize SUV, while VW sells an oddball turbodiesel V10 in its Touareg sport-utility. Note that all of these are 42-state vehicles, meaning you can't buy them new in California, Connecticut, Maine, Massachusetts, New York, Pennsylvania, Rhode Island or Vermont, as these states observe even more stringent standards.

Price: Because of the extra technology that goes into making diesels perform well on the road and comply with emissions standards, they can be an expensive option compared to similarly powerful gasoline engines. This is especially true of heavy-duty trucks — checking off the diesel option adds about $7,000 to the bottom line. The diesel models from Mercedes-Benz are the exception to the rule, as they usually command only $1,000 more than the company's gasoline V6 models.

Engine/Fuel Economy: In the past, diesel engines were written off as noisy and smelly, but technological advances over the last decade have eliminated these drawbacks (though you still shouldn't expect whisper-quiet operation in a big diesel pickup). Always paired with a turbocharger, modern diesels provide substantial torque at low rpm, which makes them an excellent choice for towing and hauling. The extra torque also offsets diesel engines' comparatively lower horsepower numbers (resulting from the fact that due to their higher operating temperatures and heavier parts, these engines cannot physically spin at high rpm). The other big advantage diesels have over their gasoline counterparts is superior fuel economy. This makes them a good alternative for SUV and truck buyers seeking lower fuel costs and extended driving range.

Safety: There is no difference in the safety of a diesel and a normal gas-powered vehicle. All the safety features available in regular gasoline cars, such as antilock brakes, stability control and side airbags, are available in their diesel counterparts.

Luxury Features/Convenience: In the U.S., diesel cars and trucks have traditionally been utilitarian in nature, but this has begun to change. Jeep only offers a diesel V6 on the high-line trim levels of the Grand Cherokee, and the diesel Mercedes and VW models can be outfitted every bit as opulently as the companies' gasoline models. Much the same is true of the heavy-duty trucks. And buyers seeking amenities like leather upholstery, DVD navigation and auxiliary audio inputs should have little difficulty getting a vehicle outfitted according to their requirements. Full-size vans remain spartan in terms of luxury amenities, but the contractors who typically buy these vehicles will find numerous convenience options aimed at work-site use.

Passenger Capacity/Interior Space: Interior space varies depending on the model you select. Rear-seat room can be tight in the five-passenger Jeep and VW models, but the midsize E320 Bluetec sedan and ML320 CDI sport-utility accommodate four adults in comfort. Families needing an extra row of seats will find both the six-passenger R320 CDI wagon and the seven-passenger GL320 CDI SUV sufficiently spacious in all three rows. If you need more than seven seats, you'll need to look to one of the full-size passenger vans ? our pick would be the Dodge Sprinter, whose high-roof option allows adults to walk around in the cabin. Heavy-duty pickups can seat anywhere from three to six people, depending on whether you choose a regular-, extended- or crew-cab model.

Luggage Capacity: The smallest diesel, Mercedes' E320 Bluetec sedan, still offers a generous 16 cubic feet of trunk space. The midsize diesel SUVs offer around 70 cubic feet of cargo capacity, while the larger GL-Class tops out at 83 cubic feet. Roomiest of all the light-duty passenger vehicles is the R-Class wagon, which provides up to 85 cubic feet of load space. Those needing more space should opt for a full-size van; here again our pick would be the Sprinter. Heavy-duty pickup buyers should assess how much cargo space (both open and enclosed) they require when choosing a cab and bed configuration. Those planning to hook up a fifth-wheel trailer will almost certainly need the long-bed option.

Manual vs. Automatic Transmission: Most diesels come with either a manual or automatic transmission. The majority of drivers will prefer the ease of an automatic, and oftentimes, there's little difference in the mileage numbers. Additionally, buyers should be aware that diesel cars with a manual transmission require the driver to shift more frequently than in gasoline cars because of the narrower rpm band in which the engine operates.

Operating Costs: The main attraction of diesels, at least in light-duty vehicles, is that they reduce fuel costs through increased efficiency. In the U.S., however, the cost of diesel fuel fluctuates wildly, and at times, diesel fuel is more expensive than gasoline. For consumers who choose to fill up with biodiesel, fuel costs will be even higher. In the long run, though, most diesel owners will enjoy a savings over comparable gasoline models (and even some hybrids). Another potential consideration for very high-mileage drivers is that diesel engines historically have a longer service life than gasoline motors due to their simpler design and more robust construction.