Dear Umbra,

I’ve always heard bad things about diesel fuel. However, I know someone who has a diesel VW that gets 50 miles to the gallon. I’m wondering if you could do a cost-benefit analysis for me. I know I can’t afford a hybrid anytime soon, and was wondering if it would be better to buy a used diesel car that gets excellent gas mileage or a regular used car that gets in the 30 to 40 mpg range.

Nelson, N.H.

Dearest Anne,

Unless you can get alternative diesel fuel, stick with a standard gasoline car.

Is it worth it?

Photo: Kent Chilton.

Regular readers will predict my research maneuver here: we go right back to my mad crush on the Union of Concerned Scientists. (Ooh la la!) Their Clean Vehicles site is a hotbed of romance, where we can find such steamy novellas as “The Diesel Dilemma: Diesel’s Role in the Race for Clean Cars.” I learned quite a bit from this page-turner.

Here’s a summary: Diesel engines go farther on a gallon of fuel than standard gasoline engines because of their design, and because of the higher energy density of a gallon of diesel fuel. But it takes more oil to manufacture a gallon of diesel than a gallon of standard gasoline, and the production and refining processes for diesel produce more heat-trapping gases. So when you’re considering the relative merits of diesel and non-diesel cars (like your friend’s VW and your regular alternative), UCS suggests knocking the mpg estimates for the diesel car down by 20 percent to account for those impacts. Since a diesel vehicle will also cost you more, you’ll get more bang for your buck from an efficient gasoline car if you’re concerned about fending off global warming, UCS says.

And the nasty rumors about diesel are true: It’s less refined than gasoline, aka dirtier. Diesel cars emit substantially more particulate matter and NOx, both of which are serious air pollutants and health hazards. Current passenger diesel engines are more polluting per mile driven than gasoline models. And no diesels currently make it into the American Council for an Energy-Efficient Economy’s Green Book.

Now, using biodiesel will improve this situation, and remove you from U.S. oil policy as much as is possible in a passenger vehicle. If biodiesel is available nearby, you will still need to closely examine whether a diesel is the car for you: Given your budget, which models can you actually afford? What is their mpg? Will the engine block need to be plugged in during the winter months, and is this a deterrent for you?

If you would like to understand the details better or hear about the future of diesel regulations and technologies, read UCS’s report [PDF] or report summary [PDF], which contain shocking plot revelations. For example, Europe is not perfect. In fact, though assumers such as myself may have thought, “[Lots of diesels in Europe] + [Green Europe beats stupid U.S.] = [Diesel good],” this is an erroneous leap. The true equation is: “[Euro-subsidized diesel] + [Mediocre Euro tailpipe regs] = [Diesel still bad].”