Should you buy a hybrid car, or just hold on to your old one?

Behaving in an environmentally friendly way can be harder in practice than in theory. This is particularly true when it comes to cars. I had an old Vauxhall Carlton which was not very energy efficient, but which did last for 12 years and over 200,000 miles. Should I have kept it that long, or would it have been better to trade it in early and get something more energy efficient. At this time I was considering this question - a few years ago - it was difficult to find any solid information. In fact it was hard to even find a good theoretical framework for thinking about the question. My search is now over - must remember this next time I change the car!. Have a look at this article if you want to know how to think about the problem. Or, if you can't be bothered, just use your car less and your bicycle more :-)