The carbon footprint of a new car:

6 tonnes CO2e: Citroen C1, basic spec

17 tonnes CO2e: Ford Mondeo, medium spec

35 tonnes CO2e: Land Rover Discovery, top of the range

The carbon footprint of making a car is immensely complex. Ores have to be dug out of the ground and the metals extracted. These have to be turned into parts. Other components have to be brought together: rubber tyres, plastic dashboards, paint, and so on. All of this involves transporting things around the world. The whole lot then has to be assembled, and every stage in the process requires energy. The companies that make cars have offices and other infrastructure with their own carbon footprints, which we need to somehow allocate proportionately to the cars that are made.

In other words, even more than with most items, the manufacture of a car causes ripples that extend throughout the economy. To give just one simple example among millions, the assembly plant uses phones and they in turn had to be manufactured, along with the phone lines that transmit the calls. The ripples go on and on for ever. Attempts to capture all these stages by adding them up individually are doomed from the outset to result in an underestimate, because the task is just too big.

The best we can do is use so-called input-output analysis to break up the known total emissions of the world or a country into different industries and sectors, in the process taking account of how each industry consumes the goods and services of all the others. If we do this, and then divide by the total emissions of the auto industry by the total amount of money spent on new cars, we reach a footprint of 720kg CO2e per £1000 spent.

This is only a guideline figure, of course, as some cars may be more efficiently produced than others of the same price. But it's a reasonable ballpark estimate, and it suggests that cars have much bigger footprints than is traditionally believed. Producing a medium-sized new car costing £24,000 may generate more than 17 tonnes of CO2e – almost as much as three years' worth of gas and electricity in the typical UK home.

Interestingly, the input-outpout analysis suggests that the gas and electricity used by the auto industry itself, including all the component manufacturers as well as the assembly plant, accounts for less than 12% of the total. The rest is spread across everything from metal extraction (33%), rubber manufacture (3%) and the manufacture of tools and machines (5%) through to business travel and stationary for car company employees.

The upshot is that – despite common claims to contrary – the embodied emissions of a car typically rival the exhaust pipe emissions over its entire lifetime. Indeed, for each mile driven, the emissions from the manufacture of a top-of-the-range Land Rover Discovery that ends up being scrapped after 100,000 miles may be as much as four times higher than the tailpipe emissions of a Citroen C1.

With this in mind, unless you do very high mileage or have a real gas-guzzler, it generally makes sense to keep your old car for as long as it is reliable – and to look after it carefully to extend its life as long as possible. If you make a car last to 200,000 miles rather than 100,000, then the emissions for each mile the car does in its lifetime may drop by as much as 50%, as a result of getting more distance out of the initial manufacturing emissions.

When you do eventually replace your car, it obviouslty makes sense to do so with a light, simple and fuel-efficient model: that way you'll be limiting both the manufacturing and the exhaust-pipe emissions. But before you buy, look into car clubs, especially if you live in a city centre: you may save lots of money as well as reducing the number of cars that need to be produced.

Of course, the exact benefits of new versus old cars, diesel versus hybrids, car clubs versus owning, and so on, are different for each person. To find out the greenest choice for you, check out the new interactive greener car guide at Startuk.org.

See more carbon footprints.

• This article draws on text from How Bad Are Bananas? The Carbon Footprint of Everything by Mike Berners-Lee