If you're tossing up whether to buy a new or used car and the environment is an issue you want to incorporate in your purchase decision, here are some issues to consider.

I've traditionally owned old vehicles - old enough that when I was done with them, their next stop was the wreckers or recyclers. It had more to do with finances rather than anything to do with the environment.

While it's been nice to have been able to squeeze a bit more life out of a vehicle than otherwise may have occurred before it was retired permanently, there has been a cost. 

For example, my last vehicle's emissions control system basically consisted of extra spark plugs (4 cylinder engine, 8 spark plugs instead of four) - not overly effective. It was also a little heavy on gas consumption. 

By the time I disposed of it, the vehicle was nearly 25 years old and I really didn't want to sell it to anyone as I knew it had a bunch of looming problems that would cost a small fortune to fix. Luckily, the local recycling center was geared up to accept cars and they even gave me a few hundred bucks for the scrap metal value.

Whether it's better to buy a new vehicle or a used one isn't clear cut as it will depend on your circumstances; but the general guideline is if a new vehicle gets far better gas mileage than what you currently have, then a new car is a good move. Fuel economy needs to be dramatically better to offset the energy that went into producing and shipping the vehicle.

However, the same could be said of a used vehicle in good condition that also achieves better mileage, and without the new car energy payback issue, but you will need to bear in mind an older vehicle may not deliver the fuel performance as what it did when it was new.

If you're in the USA and wish to do side-by-side comparisons of the fuel economy of  your current, new or older model vehicles, FuelEconomy.gov is a good resource - it lists current day models and right back to 1984. If you already know your mileage and want to get into the emissions side of things, check out my article on calculating car carbon emissions.

In your purchase decision, you also need to consider the disposal of your current vehicle - will you sell it, recycle it or will it just end up crushed into a block and taking up space at the local landfill?

Also, go beyond the mileage and disposal issues. Also give careful thought to the application. 

My old minivan was being used for off-road purposes in country where 4x4's are commonplace; something it wasn't designed to do. This created additional wear and tear, meaning more components needed to be replaced in a shorter time than what the manufacturer intended. I probably shortened its serviceable life somewhat.

After disposing of the old van, I bought another minivan - second-hand again, but far "younger" at only 10 years old, smaller engine, better fuel economy, emissions control systems and suspension. I patted myself on the back for my purchase decision, but it turned out  it wasn't the wisest choice. One year later, due to changing road conditions that see me cut off from my patch of dirt each time it rains to any degree, I'm back at square one. I really should have opted for a new/barely used 4x4 given the major fuel economy improvements made over the past few years to these vehicles.
 
Conversely, if you're hankering after a new car and the one you have now is overpowered for your purposes, consider moving to a smaller vehicle. Buying a thumping great 4x4 for mainly driving around the suburbs and only using it a couple of times a year to head out into the boonies for a short time is a little wasteful when you can rent a purpose-built vehicle for those infrequent trips. The savings you'll make on fuel and maintenance costs will probably more than make up for the rental.
 
New or used car? It's a case of horses for courses; but in our search for a "greener" ride, we shouldn't forget that combustion engine based personal transport will never be environmentally friendly, that it's more a case of aiming for harm minimization at best. While harm minimization is admirable, harm is still being done - so it becomes even more important then that we try to offset that a little more in other areas of our lives.