Honda CRXEnlarge Photo
Ultimately, by the time the 30-mpg car is 10 years old, it's been responsible for 55.5 tons of CO2, and that amount continues to accrue at a faster rate than those of the new car.
You could run it for 3.5 years before the production and first year's use of a new car matched it--but ultimately, its 10-year lifetime produces far greater emissions than those of a new vehicle.
There are huge variables in such calculations, of course.
If your old car already does 40 mpg, for example--maybe you have a flyweight Honda from the early 1990s--then it will have produced much lower lifetime carbon emissions and the new car will take longer to catch up.
But it's worth pointing out that in terms of emissions other than carbon dioxide (CO2), that old car is far more polluting than a modern vehicle.
Less stringent emissions standards from the 1990s and before permit far more unburned hydrocarbons, oxides of nitrogen, and carbon monoxide per mile to be emitted than in today's cars.
Tailpipe EmissionsEnlarge Photo
If for some reason you trade from that old 40-mpg Honda to a modern car that does only 35 mpg, then while the older car's lifetime tally will start higher, the newer car will eventually reach a point where the older car's emissions are lower.
Again, that is, if you ignore the cleanliness of those lower tailpipe emissions.
Then there's maintenance--a newer car will typically use less oil, require fewer new parts and require less intensive maintenance than an older one. That older one will be consuming other resources, not just fuel, throughout its life.
Finally, it can't be overstated that production of a vehicle is inherently an energy-intensive process. Once iron ore is extracted from the ground, it cannot be put back--the new car is ultimately another vehicle on the road that wasn't there before.
In developed countries--the U.S., most of Europe, and Japan--vehicle population is now close to static. For several years during the recent recession, the U.S. actually scrapped more cars than were sold new, meaning vehicle population actually declined.
Traffic in ChinaEnlarge Photo
But with 1.2 billion vehicles on the planet today, and the total rising to perhaps as much as 2 billion by 2035, the overall emissions burden is clearly rising. Nonetheless, that doesn't affect your friend's old car's emissions--which remain higher over its lifetime than your new car's.
New still beats old
All the data show that a car's lifetime energy use for propulsion, meaning the fuel or energy that it consumes to move itself about, accounts for a far greater proportion of its lifetime environmental impact than its production.
It's great that your friend likes her old car. Kudos to her for keeping it running, rather than letting it rot into the earth.
There are many reasons to run an old car--character, driving sensations, looks, and the memories of the journeys you've accomplished.
And, to be fair, the car you own is usually going to be cheaper than buying a whole new vehicle--financial reasons count for a lot.
But unfortunately, running an old car because it's greener isn't one of those reasons.
New cars, in that respect, will always be better than old ones.