Some ideas are stinkier than others.

This one comes from Europe and its carmakers.   Specifically, both Volkswagen and DaimlerChrysler are planning to start selling some diesel powered SUVs in this country. Soon. The idea is to make a gesture toward improving the poor fuel mileage of SUVs.

Unfortunately, putting a diesel engine in an SUV will make it more of a social menace than it already is. (Before you start heckling and tell me to get my head out of the granola, allow me to confess that the Burns family owns a 1997 Jeep Grand Cherokee and a 2003 Toyota Prius. The Jeep is indispensable for hauling large quantities of stuff and I like it. Like having a friend with a good heart and bad manners, the Jeep doesn't get out much.)

In fact, having an SUV that is big and smelly is probably OK if you live in a large, open, windswept area and plan your life around being downwind. Certain areas in Montana, Wyoming, and the Dakotas come to mind. Unfortunately, that's not where these new vehicles are likely to go. They are far more likely to be driven by hard-pressed soccer moms who live in metropolitan areas and spend a good deal of time in heavy traffic or waiting in pick-up lines for their kids.

So all the stuff that comes out the tailpipe will go into fairly urban air.

And that makes me ask a question: what are the real trade-offs and economics?   Let's start with the math.

The new Volkswagen Touareg is supposed to have a sticker price of $43,000 equipped with a 15.9 mile per gallon 8-cylinder gasoline engine. The same car, equipped with a 19.3 mile per gallon 10-cylinder diesel engine, sells for $53,000. That's a $10,000 premium for some fuel economy. If fuel costs $1.60 a gallon it will take a mere 567,000 miles to recover the additional cost in fuel savings.

That, however, doesn't count the earning power of the additional expense. Invested in, say, ExxonMobil (ticker XOM) it would yield 2.8 percent or $280 a year. That's about enough to cover the extra fuel cost now that dividends are only taxed at 15 percent. At the end of ten years you'd have some stock instead of an aging SUV with a diesel engine. If you're pressed for income, there are several energy stocks that yield more than ExxonMobil, such as BP (ticker: BP, yield 3.5 percent) and ChevronTexaco (ticker: CVX, yield 3.9 percent).

Bottom line: pay too much extra for a diesel engine and it doesn't compute in simple dollars.

But let's not take an extreme example. Let's compare the Volkswagen Jetta GLS 2.0 and the Volkswagen Jetta TDI, both with automatic transmissions. Equipped with a 4-cylinder gasoline engine, the GLS has a sticker price of   $20,240 according to Carpoint.com. The diesel version has a sticker price of $21,420. That's only $1,180 more. Based on the mileage difference, you'll recover the additional cost in a bit less than 4 years, assuming 15,000 miles a year. In simple dollars, it's a reasonable investment.

But the real costs don't end there unless you are very good at making The Toilet Assumption--- flush it, and it will go away.

The real trouble with diesel engines is that emissions don't go away. We breathe them. While diesels score well against gasoline engines in terms of carbon emissions--- the stuff that contributes to global warming--- they are many times worse when it comes to the stuff that will kill us sooner--- the nitrous oxide, particulate, and carcinogen emissions.

According to the Environmental Protection Agencies' ratings of all vehicles, the gasoline Jetta rates a "6" while the diesel Jetta rates a "1"--- where the top score is "10." The same website (listed below) will also tell you that vehicles rated "1" put out about 4 times the amount of smog forming pollutants as vehicles rated "6" and nearly 50 times the pollutants of the few vehicles rated "10."

Dallas County, where I live, is already rated among the "dirtiest/worst" 10 percent of counties in the United States. It receives 95 percent of its added air-based cancer risk from "mobile sources," i.e. cars and trucks. You can check how you are doing in your zip code by visiting www.scorecard.org.

Bottom line: If we put a value on the air we breathe, diesel engines just don't cut it.

Related websites:   

The 2003 EPA Ratings for Cars

EPA tool for Comparing Total Emissions of Different Cars

Basic Facts on Auto Emissions

Pollution by Zip Code

Union of Concerned Scientists Report (Executive Summary)

Union of Concerned Scientists Guide to Tailpipe Standards