Not really...
Under pretty much all combinations of circumstances, new cars are a lot dirtier since it takes so much more energy to make them. No car will ever use as much energy during its lifetime as it took to make in the first place.

Furthermore, new cars are almost impossible to recycle due to the amount of nasty plastic crap. Old cars can be recycled by using the good bits in other old cars and weighing in the rest.

They're also not really any dirtier in terms of emissions. Having a catalytic converter basically just means that your car puts out less carbon monoxide (which isn't environmentally that nasty, but you don't want to breath it) at the expense of lots of hydrogen sulphide and nitrogen oxides.
(gordonjcp, Thu 10 Nov 2011, 23:40, Reply)