I think it was on this site, but somewhere in the past I saw a simple rule of thumb that, on average, over the lifetime of a car, 10% of its total energy consumption is embodied energy.

For cheap-to-make long-lived gas guzzlers (think jeeps, I'd guess) it's a smaller number; for expensive to make gas-sippers (think prius) it's a higher fraction that's embodied, but hopefully that's a place to start from.

If a car is not an emotional investment for you it's best to hold onto what you've got, per the previously two commenters. Adam makes the good point regarding the cost and benefits of improving your home versus a car purchase. Sometimes, I wish I didn't like cars as much as I do; it would make being green simpler.

I'd also chime in on the side of not replacing a vehicle for such a small improvement in mileage and such a small number of miles per year.

One thing to consider to be greener would be whether you could drive even less if you had a good bicycle. Bikes give a bit of good exercise, enjoyable transportation, rarely get stuck in traffic, and burn only as much carbon as it takes to feed you.

For anyone with any form of public transportation nearby, folding bikes can be ridden to the station on one end, taken with you, and then ridden to the final destination on the other end. This would be a good way to use the bike as transportation for longer distances than one may reasonably ride.

One more consideration: When you buy a new car, your old one doesn't come off the road--someone else buys it and drives it, and someone else drives THEIR old car...until one gets junked.

So, buying a new car means ONE MORE CAR in the world, contributing to the carbon load. I just can't justify trading in my perfectly good 16-yr-old Volvo 240 wagon (4-cyl) that seats 7 if needed, and gets up to 28 mpg on the highway...

Hi, Tom. Though I basically agree with the previous posters about the limited climate benefit you'd see by buying a new car given how little you drive, that's not the only consideration. Automotive safety technology has come a very long way in the last 20 years, beginning with the availability of 3-point seatbelts and headrests at all seating positions, the development of crumple zones to absorb crash impact energy, antilock braking systems, front airbags, seat-mounted and curtain side airbags, and vehicle stability control. Emission controls for smog-forming air pollutants have also improved a great deal in the last 20 years, so a new--or much newer--car would contribute significantly less to air pollution.

And you could buy a hybrid which would get far better than 27-30 mpg while providing the same utility as your current sedan. If you bought a distinctive hybrid like a Prius or the forthcoming Honda Insight, you'd be sending a clear signal to everyone around you that reducing your energy use is important to you. Those kinds of signals play a crucial role in social change.

In the end, of course, it's up to you to decide what your highest priorities are with regard to your vehicle and finances.

It almost is never a good idea to scrap an old car to buy a new one, in environmental terms, unless you drive more than 25000 miles per year and are willing to switch from a big inefficient vehicle to a small hybrid. A general rule of thumb is that holding on to your own car will save you about $3000 per year. How many Terrapasses can you buy with that much money? Enough to offset 100% of your old car's emissions fifty times over.

I am driving a 1982 Honda Civic Wagon. It gets 35 mpg reliably, and up 40 mpg if I keep it down to 60 on the highway (which I am still doing even as gas goes down). I would only buy a car if and when I can no longer drive the old one - it's hard to find parts these days, and I am thankful for a wonderful mechanic who tinkers and makes it work. For a while I had a 1990 Honda Civic hatchback that got 40 mpg. I loved that car.

If I were to need a new car, it would be a car with great mpg. Probably a Honda again. I also wonder why the newer cars (even some Hondas) don't the mileage of my old car. Anyone know?

Hi, Jackie. Part of the explanation for lower MPG in many modern cars is that they have much more powerful engines than your '82 Civic wagon or your previous '90 Civic hatch. Americans like rapid acceleration, and that comes at a price in lower fuel economy.

Modern cars tend to be larger than they used to be, too, so they weigh more. For example, today's Toyota Corolla is larger than the original Toyota Camry, which has grown to accomodate.

Also, modern cars have many more amenities (power everything, better stereos, climate systems, standard A/C) and safety features (headrests and 3-point seatbelts at all positions, antilock braking systems, front and side airbags, and, most recently, vehicle stability control in some models) than either of the cars you mentioned, all of which add weight. This does make them more comfortable and much, much safer, however.

Wow, lot's of great posts here... especially the one regarding a bicycle. Of all the good things to recommend, a bike is the best. Many people overlook the folder concept, but there are great ones being built right here in my own backyard (Bike Friday). We own three and my personal BF is a 'Speeding Tikit'... stealth black, fast folder, 27 speeds, sexy.

Having listened in at the recent Energy Roundup in Eugene, OR, a community forum on many sustainability issues, it becomes evident that the first post is a reliable best bet as well. With over 40% of GHG's being emitted by our buildings, we really need to button them up and reduce our footprints both at home and in our places of work.

So if you did both of these, and with the option of electric bikes if pedaling is too difficult or your terrain is hilly, adding solar pv and/or thermal, combined with efficiency upgrades, the choice of public transportation, and even walking for certain local needs... all add up to a more socially improved planet whose citizens are healthier, have more time to socialize, and slow down to at least remember the visit !

As for me and mine, we'll see you out there, on our bikes of course... JD Howell, Eugene, OR