That's cute - my 1981 Toyota Tercel still gets 35+ mpg.

I must disagree.

Edit: To clarify, I'm not poking fun at you per se, but the idea that our technology will 'always' trend toward more efficient engines is one that we've pretty much failed at until very recently, which is still arguable.

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.

A used car has already been created which is different from buying a new car every time which takes an insane amount of energy to create.

Any pointers for the "insane amount of energy"? I vaguely remember the local tech/car newspaper writing that after something like two years of driving (used 80s car v.s. brand new efficient car) the new car has spent less energy per kilometer. Or was it even less...

What I hear most often is 80% of the energy consumed by a typical car is used during the manufacturing process.

It's also worth noting that many 80's cars got very good gas mileage. My first car, an '86, got low 30's, and with a tune-up some owners got higher than 35. So, be wary of the studies you read comparing new car mpg to old car mpg. My car got better or equal gas mileage compared to everything but hybrids like the Prius and Insight, and those are not representative of modern cars. I would bet the studies you have been reading have been comparing Insights (70mpg, and actually made since 1999) to 80's Corvettes (14mpg) or something along those lines.

"A California-oriented paper http://www.environmentaldefense.org/documents/3986_CAautocar...

states that direct tailpipe emission of CO2 accounts for 68% of the average vehicle lifecycle carbon emissions, with 21 percent linked to production and delivery of fuel, and 11 percent are due to manufacturing, including materials production."

So 11%. Pretty far off.

You're talking about CO2; he's talking about energy.

Two completely different things!

The Prius is not particularly effective.

You can get an approximate answer by looking at the cost of the car. A new car costs say $30,000. Gas costs $2.75/gallon, so a new car is equivalent to about 10,000 gallons of gas.

It'll take a very very long time to recoup that.

The cost is not all energy though, so at best that is an upper limit. Furthermore there are cheaper sources of energy than gasoline (eg coal), so it is not even an upper limit.

Perhaps the rationale is that a used car requires no energy to manufacture.

This might be completely rational if there was an excess of perfectly good used cars out there, but there isn't. Cars hold their value incredibly well compared to almost any other consumer product. We're not talking about $30 DVD players. Any used car worth a damn will get sold and bought up. There is almost always someone willing to take up the slack and keep a car running for decades. Therefore, by buying a new car, you're simply decreasing the demand for used cars slightly and lowering their price, which will just mean that someone will just get a better deal.

The only way to effectively reduce carbon emissions on the production side is to drive less person-miles. Buying used or new doesn't change much.

In the US at least, efficiency improvements over the last decade or two have generally been offset by safety and performance improvements. Modern engines are more efficient, but they're also more powerful and they're driving heavier vehicles that are more likely to keep you alive in a crash. Americans drive on highways a lot and the need to be able to accelerate through an on-ramp quickly enough to feel safe has lead to cars with less net fuel efficiency than we might have.