Someone mentioned that it is greener to keep your car longer. I don't understand this because the car you trade in or sell will eventually be driven by someone else, right? So what's greener about you driving the car vs. someone else? Please explain, as I have always wondered this.

 

Most of the time older cars will not be driven any longer, and instead go to the junkyard and only a few parts will be removed. Assuming that ever car that was sold was kept and driven into the ground, then it wouldn't matter, but mostly good cars are still showing up in droves at these junkyards.

 

If everyone kept their cars longer, on a theoretical basis, fewer cars would be manufactured-therefore using less energy, therefore greener. That's the theory, but that assumes car makers would go along with that scenario. Being capitalists, I suspect they would not settle for fewer sales without some counter-action on their part. (Lower quality parts leading to earlier vehicle failures comes to mind).
Lower sales would also lead to fewer autoworkers employed.

 

It's not. Approximately 15% of the lifecycle energy cost goes into production and disposal assuming a 95% metals recycling rate and a 50% plastic recycling rate. You can read the details here.

Take a look at table 5.4 on page 112 of the pdf. Do yourself a favor and read the entire document. It's a little dated but a very good primer.

The numbers would say that if you can reduce the carbon impact of your operating cycle by greater than 15% (or approximately thereabouts) you should upgrade your car.

 

Hmmm... the numbers are based on an average life-cycle per vehicle being 300,000 km (~180,000 miles). So that 15% is the same energy as driving 45,000km (~28,000 miles). I think the calculation is a bit more complex than saying a 15% improvement in carbon emissions is reason to upgrade.

 

Fair enough. I was paraphrasing too literally. Double it to 30% if you like. I think the study at least pretty clearly shows that continuing to operate a vehicle of lesser efficiency just because it will run will not minimize your carbon impact and that moving from a car that gets 20mpg to a car that gets 35mpg is desirable if you have the means to do so.

 

That makes sense ot me. My last car i bought new and drove for 20 years and then got rid of it. I think i got the most out of it that was possible.......without spending several thousands repairing it. I thought it was the right thing to do. I bought my TCH after that.

 

First, 20->35mpg is an improvement of 75%, not 30%. Even then, going from 20mpg to 35mpg wouldn't reach a break even until the new car had been driven for 64,000 miles.

However, the energy and material requirements from the PDF are a projected estimate of a car built in 2020. So all of these calculations are at best rough estimates.

I think the take away is that even with a substantial improvement in fuel economy (and 20 to 35mpg is substantial) a vehicle needs to be used for a substantial amount of time for it to offset its creation costs.

 

I'm aware of how the math works. 20 to 35mpg was a realistic scenario for many drivers today. It is also highly likely that 64K would be reached, so I'm not sure what your point is. Your conclusion regarding the "roughness" of the data is a non-sequitur. Can you support your conclusion? Most vehicles are used for a substantial amount time so again, I'm not sure what your point is.

 

I wasn't questioning your ability to do the math. I only pointed out the discrepancy because you started with the statement that a 30% improvement justified a vehicle replacement, but then gave an example of 75% improvement.

Typically when I give numbers, I like to point out if my estimates are likely to be off. In this case, I've assumed that we are close to the 2020 target for vehicle composition, which could at best be called aggressive. For example, they assumed the use of magnesium would double over 20 years as vehicles became lighter in weight. However, a casual search for data says the average curb weight for cars increased from 3,050 to 3,240 from 1995 to 2004. So it would seem our MIT researchers assumed the auto industry was headed in a different direction.

So is production a greater or lesser percentage than the MIT research indicates? I don't know and I don't claim that I can figure it out after a few hours of reading and some casual research.

You're absolutely correct that 64k is likely to be achieved by most vehicles. If the goal is to be greener, though, people need to keep in mind that each new vehicle needs to be dramatically better if the life cycle is short.