The eco-friendliness of a car does not come down to its MPG only. You also need to look at how much greenhouse (and other) gases come out from the combustion. New cars tend to do better.

But on the other hand, building a new car requires lots of energy, so maybe in the end we're better off driving old ones... ? It's all so complex, I don't know what to think, personally!

If we're talking about greenhouse gases, then MPG is the only important measure. For one, the vast majority of the gases coming out of the tailpipe are going to be CO2 and H2O (aside from pre-existing constituents of the atmosphere). All of the other gases aren't going to have a significant impact on the greenhouse effect.

So to a very close first approximation: MPG = carbon emission per mile = greenhouse impact per mile.

Indeed, a more polluting vehicle (say a really dirty old diesel engine) would if anything have less of an effect on the greenhouse. Since it's particulate emissions would promote an offsetting cooling effect.

Are you certain about that? I know a car emits much more CO2 and H2O than most anything else, but as an example, I was pretty sure the smog over L.A. was a direct result of oxides of nitrogen. Or was that only toxic, and not greenhouse, and thus not part of our consideration?

According to http://www.epa.gov/oms/climate/420f05004.htm cars also produce significant amounts of methane and nitrous oxide, which are both greenhouse gases. However good old CO2 represents 94-95 percent of the greenhouse potential from your average car.

So no, it is not all CO2, but the rest is a rounding error.

Precisely. If you're talking about greenhouse impact, MPG is the only important measure.

If you're cared about pollution aside from that then yes, there are huge differences depending on make, model, maintenance, etc.

Should the water vapour also be an important part of your cars greenhouse gases?

(See http://en.wikipedia.org/wiki/Greenhouse_gas#Role_of_water_va...)

Nitrogen oxides are both toxic and greenhouse gases, but in the quantities that they're emitted from tailpipes, the toxicity is a much bigger problem than the greenhouse effect.

Pft, the pistons in that car were about as big as a coke can, and there were four of them, plus it had a recirculating exhaust system.

It was also mostly made out of aluminium, much to the chagrin of my parents / loved ones.