car at the NAIAS - clean dieselGerman automakers like Volkswagen, BMW and Mercedes, and Japanese automaker Mazda said this week at the North American International Auto Show that they are planning to offer more vehicles that run on "clean" diesel.

U.S. automakers, though, say they won't offer diesel engines in anything but heavy-duty work trucks because they don't see consumer demand, nor, they say, can they price their vehicles high enough to pay for diesel systems.Diesel engines are more powerful and fuel-efficient than similar-sized gasoline engines (about 30%-35% more fuel efficient), notes fueleconomy.gov. They also provide greater acceleration than most four cylinder engines. But they also cost more than gas-powered cars. And some people just aren't aren't comfortable driving a car that can't be fueled up at every gas station, as diesel fuel, while widely available, is not at every station.

Currently, there are 13 diesel models offered in U.S. showrooms: Volkswagen Golf TDI, VW Jetta TDI, VW Jetta Sportwagen TDI, and VW Touareg TDI; BMW X5 xDrive35d, BMW 335d, BMW 550i; Mercedes-Benz E350 Bluetec, Mercedes ML350 Bluetec, Mercedes R350 Bluetec; Mercedes GL350 Bluetec; Audi Q7, Audi A3. Mercedes later this year will add an S Class diesel. Mazda plans to bring a diesel version of its Mazda6 to the U.S. in 2012 that is expected to get 43 mpg. And Volkswagen says it will produce a TDI diesel version of the new Passat it will begin building in the U.S. later this year.

Ford, Chevy and Chrysler all offer "Super Duty" pickups that run on diesel, as well. Those heavy work trucks need the diesel engines to handle the extra payload weights they carry, as well as for towing trailers and the like.

Today's diesel cars are known as "clean diesel" because they run on low-sulphur diesel that is now pumped at every station that sells diesel fuel, and because the vehicles are equipped with after-treatment systems that make the tailpipe emissions cleaner.

So who is right? Are diesels worth it, or not worth it?

Consider the Volkswagen Jetta Sportwagen TDI, which Consumer Reports recommends, as an example.

The 2011 VW TDI is rated at 30 mpg city/42 mpg highway, for a 34 mpg combined rating. The gasoline version is rated at 23 mpg city/31 highway for a combined rating of 27 mpg. Based on government ratings, that puts the TDI at 26% more fuel efficient than the gas version of the same car. Diesel car enthusiasts often report getting even greater mileage than the government rating, especially on the highway.

But there is more to the comparison. The government website currently estimates the current fuel costs per year for the TDI at $1,469 based on typical driving. The annual cost for the gas version is $1,715. That is based on a current national average for diesel of $3.33 a gallon and $3.09 for regular unleaded gas. That means that, based on averages, it could cost about 8% more to fill the tank with diesel than gas.

Today's diesel engines are much quieter and smoother than those automakers put in cars in the '70s and '80s, and they are certainly more refined than those found in big-rig diesel truck engines of today.

But there is still more to consider before choosing a diesel car. The gap in price between gas and diesel can vary regionally based on demand, and will impact the ongoing cost of owning one. Sometimes the spread is a few pennies in one part of the country, and sometimes it can be a few dimes. You can research the trends at the U.S. Energy Administration website.

Then there is the actual price of the car. The TDI costs $2,167 more than the gas version similarly equipped. If going by pricing alone, an owner would have to drive 10 years to make the money back based on government averages.

But you can't go by government averages alone. If you drive many more highway miles than city, the fuel savings with the diesel will be more. Likewise, if the spread between diesel and gas tends to be lower than the national average where you live, the savings will be greater as well.

There are two more considerations when evaluating a diesel car. Diesel engines are notoriously longer lasting than gas engines. It is not uncommon for owners of VW and Mercedes diesels to boast better than 200,000 miles and up on their cars. And there is driver satisfaction to consider. The acceleration on a diesel, and the feel of driving it, gives the enjoyable sensation of driving a larger, more powerful engine, like a V6 or V8, but without the gas guzzler penalty.

Executives at Ford, GM and Chrysler say they are reluctant to offer diesels in their cars because they don't believe their customers will pay the premium prices needed to cover the costs of the diesel engine and the after-treatment system necessary for emissions compliance. But German automakers, including VW, say they not only have their costs lower than Detroit automakers, but they also are able to get higher prices and make bigger profits per vehicle from buyers across the board.

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