I was wondering -- are there not tradeoffs for driving an old (mechanically maintained) motor car rather than a new, fuel-efficient, and mostly recyclable one? I use older cars which I consider to be a sustainable utilization of resources (for the record -- my seldom-used long-distance cruiser is 25 years old while the little run about is 37 years old).

I am battling with the suspicion that in the name of environmental preservation car manufacturers are promoting conspicuous, rather than conscientious, consumption - and many folk are rushing to reduce fuel consumption at the expense of additional primary and secondary resource consumption.

Gil: I'm not inclined to ascribe motives to anyone else, but you've put your finger on an important question. A new car is not only a financial investment; it's also an energy investment; think of it as the "embodied energy" investment that's used to produce the car, its component parts, and to extract, refine, manufacture and ship everything that goes into. Just as the financial investment generates a return in the form of lower fuel expenditures, the embodied energy investment generates a return in the form of lower energy use.

I don't have enough data to provide a quantitative answer -- you didn't say the fuel efficiency of your present vehicles, but we can guess that you're likely to keep a new car a long time. But here's how researchers Jonathan W. Fox and David R. Cramer explain it in their 1997 report Hypercars: A Market-Oriented Approach to Meeting Lifecycle Environmental Goals:

A lifecycle energy analysis in the 1970s showed that making a typical automobile from virgin materials used about as much energy as each year's driving: i.e., the ratio of embodied energy to total lifecycle energy, assuming a 12-year product life, was ~1/13. A 1995 assessment by Ford's Scientific Research Laboratories found little change: the embodied materials and manufacturing energy was equivalent to 1.2 years' driving (or 1.4 years counting the "energy overhead" of gasoline production).

(By the way, Fox and Cramer also note: "Thus during 20-odd years, while fuel economy doubled and more energy-intensive light metals were more widely adopted, improved industrial efficiency and recycling roughly compensated.")

I don't know if the relationship holds for hybrids, but let's assume it does. And let's assume you keep your new 50 miles per gallon hybrid for only six years (probably optimistic), and assume that your old car gets 20mpg (almost certainly optimistic). In that scenario your embodied energy investment in the new car is yielding an annual energy ROI of about 60%; in other words, your energy investment is paid back in about 20 months, and beyond that it's gravy.

(How long you should keep that new car depends on how much and how quickly they improve.)

Of course energy isn't the only benefit. Modern vehicles emit far less air pollution -- and greenhouse gasses -- and are generally safer (though harder to maintain yourself), than your 25- and 37-year-old clunkers. And the energy they save are today's oil imports.

So is investing in a new car or maintaining an old car the best investment of money or energy? I actually struggled through this question recently, in deciding to buy a new Prius. (And if you favor the "keep 'em running" strategy, I'd got a very nice 99 Saab I'd be happy to sell you…)

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Gil Friend, systems ecologist and business strategist, is president and CEO of Natural Logic, Inc. -- offering advisory services and tools that help companies and communities prosper by embedding the laws of nature at the heart of enterprise. Sign up online to receive his monthly column via email. Read Gil's blog here.