Dear Umbra,

I have the opportunity to convert a 1979 non-turbo Mercedes into a non-diesel. The question I have is: to which should I switch, biodiesel or vegetable oil? I can’t seem to find out which one is best, just that these two are better than any petroleum-based fuels, which we already know (thanks for your earlier column on hybrids vs. veggie oil). Also, is it possible to switch from biodiesel to veggie oil, or vice versa, once a conversion is made?

Cincinnati, Ohio

Dearest Margaret,

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I’ve noticed a lot of confusion about the distinction between straight veggie oil (SVO) and biodiesel (BD), so I’m happy to continue recapping.

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It’s so easy, even a fuel can do it.

© Corbis.

Here’s the lowdown: No conversion is necessary to run your diesel Mercedes (or Peugeot or VW or Ford) on biodiesel. As I’ve mentioned before, biodiesel has been chemically altered to behave almost exactly like petrodiesel, and it can go straight from pump to tank. (Learn more from friendly Rob, this week’s InterActivist.)

But veggie oil is different. Although you can put SVO directly into your diesel tank, it’s not generally recommended. SVO conversion kits essentially add a second fuel tank, and your old tank remains a necessary part of the action. To address your second question, then: switching back and forth is simple if you do this conversion, since you could fill your regular tank with BD.

Which you should choose for your primary fuel is mostly related to your financial and time constraints. If BD pumps are available where you live, you’ll pay market rates, which — according to the government’s latest alternative-fuel price report — were hovering just under $2.50 per gallon this spring. (Lately, my faithful readers have been reporting prices closer to $3.50.) Eventually you may need to replace some hoses, but other than that it’ll be exactly like using petrodiesel. Converting to SVO, on the other hand, involves the price of the kit and the cost of installation; it’s an endeavor that could run you as much as $2,500.

The ongoing hassle of SVO is obtaining and preparing the oil, and repairing the conversion kit if it malfunctions. Instead of stopping at the BD pump, you’ll need to plan in advance to obtain, perhaps heat, and definitely filter and store your oil. So although waste oil often involves no cash, it does involve time. Thus far, SVO loses to BD on the hassle front. But to look at it from another perspective, using SVO is much less hassle than making your own biodiesel — a daunting task.

In a way, it depends on your goals. Do you want free and/or dirt-cheap fuel? Go with SVO or make your own BD. Are you attracted to home chemistry projects and excited to transesterify fatty acids? Definitely make your own. Do you want to reduce your emissions but change nothing else about your car habits? Go with BD at the pump. Is there no fueling station near you? See if you can form a biodiesel co-op with friends and neighbors. And happy motoring.