My husband is an environmentalist. He wants one of the new alternative-fuel cars, but I have a dream to surprise him for his birthday with a 1970 muscle car (a Malibu or something like that) from his youth, which I would retrofit with a biodiesel or natural-gas engine. My questions are:

1. Should I put in a biodiesel engine or a natural-gas engine?

2. Who can do this for me in California (preferably the Bay Area)?

3. How much should I expect to pay to have this done?

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Belvedere, Calif.

Dearest Lucy,

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I’m glad you wrote before rushing out to buy that Malibu coupe. Your idea is great, but the execution will be incredibly difficult, if not impossible, and may end up disappointing your environmental hubby to boot. So let’s put the project in park for a moment to ponder it.

Greening up a muscle car isn’t easy.

Newer alternative-fuel cars have multiple attractions. One: showing marketplace support for fuel innovation. Another: reduced carbon-dioxide emissions. A third: high miles per gallon for whatever fuel is in the tank, which translates into needing to fuel up less frequently (although apparently this is not the case with compressed-natural-gas [CNG] vehicles). Still another attraction, which may or may not matter to your husband, is owning a newer car that runs nicely, has nifty amenities like a CD player, and projects a bourgeois eco-image.

In your plan, only the marketplace support for fuel innovation and the reduced carbon emissions would be achieved — and that through the purchase of fuel, not of the vehicle. So think about why your man wants an alt-fuel car, because if you go to the massive trouble of converting an old car and it doesn’t suit his fantasy, you’ll both be very sad. An old car will get lower mileage, will have wear inside and out, and may not have delay windshield wipers and all that good stuff. On top of that, because it’s been around the block quite a few times, it is likely to require more repairs, or at least require them sooner. (With new cars, we can maintain the fantasy that they will not need repairing at least for a little while.)

If you do decide to convert the old car, go talk to a good, friendly mechanic. Engines aren’t one-size-fits-all. The physical dimensions of an engine and its power need to work with the rest of the car. Even if those two factors match, installing an engine for which the car was not designed will probably be incredibly expensive. A friend of mine wanted to switch her VW Westfalia over to a diesel engine, and she was told it would cost upwards of $6,000 — and probably much more. Modern CNG vehicles have specific safety standards to protect against fires and methane release. I’m not sure a ’70s car would qualify — and in any case, the higher cost of CNG autos is attributed to $6,000 fuel cylinders. Your idea, in short, while possibly priceless, is not cheap.

My advice in this instance, and that’s what I’m paid for, would be to look into diesel cars from the ’70s, which you can run on biodiesel. You’re on the West Coast so many such cars will be in decent shape — not to mention far cheaper than car + engine. It’ll be less of an investment, and you’ll still be able to send a hipster message on your ice-cream outings. And, hey, if he wants to, hubby can still get a newer car for his main wheels.