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HybridCenter Q&A: To Trade or Not to Trade?
8/03/06

Interesting question that I thought might be more widely informative:

Q: A friend of mine drives a 25-year-old gas guzzler.  His mileage is terrible, but he thinks he's probably doing the ecologically right thing by holding on to his old car, given the resources and energy required to make a new car.  I had never considered this angle, which seems crucial to acting in an environmentally responsible way.

I wonder if either of you have ideas about this or know of a resource to consult to help one assess the full impact of one's consumption.  Trading your old station wagon for a Prius, or recycling your old air conditioner and buying a newer, more efficient model, is it really saving energy? 

A: A variety of reputable investigators have concluded that 85-90 percent of energy use and global warming emissions attributable to an average vehicle over its entire lifecycle come from operation. Only 10-15 percent is production and disposal. This is true for both hybrids and conventional vehicles.

In order to achieve a net reduction in per-mile global warming emissions, (i.e. to offset the additional emissions from manufacturing and disposing of another vehicle) the new vehicle will have to get 10-20 percent better fuel economy than the old vehicle, assuming the vehicle will be driven in a typical way (i.e. that it will be used for its full useful life - usually around 170,000 miles).

Or, to look at it a different way, how long would it take to offset the manufacturing and disposal energy associated with the new car?

Assuming 15 percent of lifetime energy use for the new car is manufacturing and disposal, and a lifetime of 170,000 miles, you can calculate the miles which you need to drive in order to make up for the manufacturing and disposal of the new car, using the following formula:

Miles = 25,500 / (MPG_new / MPG_old - 1)

For example, suppose his old vehicle gets 15 mpg and he's considering replacing it with one that gets 35 mpg. As soon as he's driven  25,500 / (35 / 15 - 1) = 19,100 miles, he will have already made up for the energy and global warming emissions that went into producing that vehicle. For the remaining 150,000 miles of the new vehicle's life, the reductions in global warming pollution are all "profit."

That's not to say he should drive his old car another 19,000 miles. Not at all. So long as he replaces the old car with one that gets at least 10-20 percent better fuel economy, he should make the replacement right away.  In certain situations, it may take longer for fuel savings to make up for the manufacturing emissions. For example, if the driver does very little driving, it will take longer to offset the manufacturing emissions, and it may be worth discounting future emissions - something that's beyond the scope of the current discussion.

Posted by: Don 8/03/06

Original post and comments can be found on Hybridblog.org.