Q: Is it better to drive an older, well-maintained car that gets about 25 miles per gallon or to buy a new car that gets about 35 miles per gallon?

– Edward Peabody, via e-mail

A: It definitely makes more sense from a green perspective to keep your old car running and well-maintained as long as you can – especially if it’s getting such good mileage. There are significant environmental costs to both manufacturing a new automobile and adding your old car to the ever-growing collective junk heap.

A 2004 analysis by Toyota found that as much as 28 percent of the carbon dioxide emissions generated during the life cycle of a typical gasoline-powered car can occur during its manufacture and transportation to the dealer; the remaining emissions occur during driving once its new owner takes possession.

An earlier study by Seikei University in Japan put the prepurchase number at 12 percent.

Regardless of which result is closer to the truth, your current car has already passed its manufacture and transport stage, so the relevant comparison has to do with its remaining carbon footprint compared with that of a new car’s manufacture, transport, and driver’s footprints – not to mention the environmental impact of disposing of your old car or selling it to a new owner who will continue to drive it. There are environmental impacts even if your old car is junked, dismantled, and sold for parts and scrap.

Don’t forget that the new hybrids – despite their lower emissions and better gas mileage – actually have a much larger environmental impact in their manufacturing compared with nonhybrids.

The batteries that hybrids use to store energy for the drivetrain are no friend to the environment – and having two engines (gas and electric) under one hood increases manufacturing emissions. And all-electric vehicles are only emission-free if the outlet providing the juice is connected to a renewable energy source, rather than a coal-burning power plant, which is more likely.

You can assess your car’s fuel efficiency or emissions by using one of many services available online. The government website FuelEconomy.gov provides fuel efficiency figures for hundreds of different vehicles dating back to 1985.

Web­­sites like TrackYourGasMileage.com and MPGTune.com can help you track your mileage and provide ongoing tips to improve fuel efficiency for your specific make and model vehicle.

MyMileMarker.com takes it a step further, making projections about annual mileage, fuel costs, and fuel efficiency based on your driving habits. If you have an iPhone, you can keep track of your car’s carbon footprint with the new Greenmeter App from Hunter Research and Technologies. The program tracks numerous variables to make its calculations as you drive, including weather conditions, cost of fuel, vehicle weight, and more.

If you simply must change your vehicle, be it for fuel efficiency or some other reason, one option is to simply buy a used car that gets better gas mileage than your existing one.

There’s much to be said, from many environmental vantage points, about postponing replacement purchases – of anything, not just cars. It’s a good idea to keep out of the waste stream anything that has already been made and to delay the additional environmental costs of making something new.

Got an environmental question? Write: EarthTalk, c/o E – The Environmental Magazine, Box 5098, Westport, CT 06881. Or e-mail: earthtalk@emagazine.com.