All's Well That Ends Oil

Either choice is a good one as long as you continue to find alternatives to driving, and your car-n-fuel expenses do not prohibit other environmentally beneficial acts (e.g., insulating your house). These guidelines apply to everyone, not just you. I usually mention them at the end, but thought I'd try setting them up front.

A good passenger diesel car will get about 34 mpg, according to the EPA, and about 50 mpg or more according to the word on the street. A good gas-electric hybrid, the Prius, will get in the mid-40s mpg, according to both accounts. Gasoline emits about 24 pounds of CO2 per gallon, regular diesel about 28, biodiesel (B100) about 7.3, and your 20-80 mix (B20) around 23. We can do a little math to compare the emissions from two cars: using the EPA's handy charts, I chose the '06 diesel VW Bug to go up against the Prius. In 100 miles at 45 mpg, the Prius emits 53.3 pounds of CO2. Over the same 100 miles, the 34 mpg Bug will release 81 pounds on diesel, 21 pounds on B100, and 67 pounds on B20.

To go further with this little cul-de-sac of carbon math, you would then figure out how many months and miles of B100 vs. B20 per year for your climate. Compare those to the emissions from yearly gasoline miles of your chosen hybrid, and there you are. (I'm not going to bother with the further calculations, as we would be delving too far into the theoretical.) Diesel fuels are also responsible for significant particulate matter emissions, which are almost all reduced with both B100 and B20. Nitrogen oxides are slightly increased -- here's a handy chart [PDF]. The Biodiesel Board also provides an interesting Emissions Reduction calculator for diesel fuels.

That's the emissions roundup, but as to which will be better for the environment ... Down here in the stacks it's always Clark Williams-Derry Appreciation Week, and Clark has interesting things to say about car comparisons. Here is a thought-provoking Clark piece about the role of spending money in car choices, which you may enjoy: Is it better to put money into a car (and its expensive fuel) or keep the money in your pocket for that insulation project?

As I said at the outset, I think both your choices are good, with the added benefit of pushing the car economy toward more efficient private vehicles. Just avoid plain old diesel, whatever you do.

Asthmatically,
Umbra