**Update:** Kaurinui does an interesting bit of arithmetic using the numbers provided here, and finds that the manufacturers that seem to be the most polluting, pollute the least. I'm not much of a mathematician, but this reasoning makes sense to me (if GM manufactures 36.97% of all US cars and produces 31% of the carbon, that's actually better than Nissan, which produces 2.87% of the cars, but produces 5.00% of the carbon.) Would those of you who are good at math care to take a look and comment on these results?

**Another update:** Will does the math and comes up with a very different scenario. The ranking, by Will's calculation, is *completely* different from Kaurinui's.

While this article raises some interesting and, I believe, important points about the relative amounts of carbon pollution produced by various sources, regions and population sizes, a question it provokes but fails to address is, "Which fleet of cars built by the six manufacturers named is least or most polluting on a fleetwide average-per-vehicle basis?"

Phrased another way, what we're trying to learn is, which manufactuer's fleet, on a number of vehicles-weighted basis, produces the most carbon pollution and which produces the least. Answering this question, USING THE NUMBERS PROVIDED IN THE ARTICLE (assuming they're correct) isn't tough, it's just a bit of arithmatic. Here we go:

We're going to build an index in which zero, "0", is the mean fleetwide carbon polluton released by all listed manufacturers.

Deviation above that mean = more polluting.

Deviation below that mean = less polluting.

According to the article, here are the raw numbers:

GM: Cars, 36.97% Carbon, 31.00%

Ford: Cars, 28.59% Carbon, 26.00%

DaimChry: Cars, 17.45% Carbon, 16.00%

Toyota: Cars, 10.68% Carbon, 9.00%

Honda: Cars: 3.44% Carbon, 6.00%

Nissan: Cars, 2.87% Carbon, 5.00%

Remembering that in our index deviation from the all-manufacturer mean of zero, "0", indicates more or less polluting than the total fleet, here are the marginal deviations from the mean for each manufactuer's fleet (note: the smaller the number, the less polluting that manufacturer's fleet is on a size-weighted basis):

GM: -0.06

Ford: -0.03

Daim/Chry: -0.01

Toyota: -0.02

Honda: 0.03

Nissan: 0.02

The results are counter-intuitive, with the GM fleet coming in as the least polluting on a size-weighted basis and Honda coming in as the most polluting on a size-weighted basis.

What this shows is that sometimes even well-intentioned or supposedly objective reporting can unintentionally create a misimpression ~ the misimpression here being that the "big three" automakers' cars are the most polluting.

Surprise, surprise, they're not. Honda's are. Who wudda thunk it?

Please feel free to try this at home ;~)

I'd like to see Kaurinui provide a couple of the hidden steps in the "bit of arithmetic" he uses. Specifically, what's the technique for establishing the index and what is the calculation for deviation? Intuitively, I believe the comment and conclusion are probably accurate.

OK, I've done the math and found slightly different results.

The first thing I did was put the data into a spreadsheet: the manufacturer, the number of cars manufactured, and the % of total emissions.

Then I took the number of cars manufactured, totalled it up, and divided each manufacturer's share to get % cars manufactured. I then calculate the % emissions divided by the % cars to get an idea of emissions per car.

It wouldn't display the table in HTML in this comment entry, so I posted the table here:http://www.liquididea.com/2006/07/calculating_emissions_per_car.htmlSeveral things worth noting: the % emissions don't add up to 100%. I don't know where the missing 7% is, but it's not in the source data in the article sidebar.

The last column (see table at link above) indicates roughly emissions per car. Of course, these are approximate because the source data rounded percentage of emissions to whole numbers. But looking at the emissions per car, we see that lowest to highest looks like: Honda (0.84), GM (0.89), Toyota (0.90), Nissan (0.93), Ford (0.97), DaimlerChrysler (0.98).

So the big surprise there is GM: How did GM manage to turn out a low emissions-per-vehicle rating?

Carbon emissions is pretty basic: the only two factors involved are the number of miles driven and the miles achieved per gallon. Any two cars burning gasoline will emit the same exact carbon emissions per gallon of gas. So either GM cars are more fuel efficient (doubtful) or they are being driven less.

Or, a third possibility is that a larger percentage of GM cars are diesel vehicles, and diesel might have a different carbon emissions profile. But I would think that the percentage of diesel vehicles would be so small as to not be significant.

So there are two mysteries here. How did GM do so well? And how did Kaurinui generate his numbers, which differ from mine by a great deal?

OK, after reading the detailed report at Environmental Defense, I redid my calculations and ended up with different results:

http://www.liquididea.com/2006/07/calculating_emissions_per_car.html

The conclusion is that the right order from least emissions per car to most emissions per car is:

Honda (0.82)

Toyota (0.94)

Nissan (0.97)

GM (0.99)

Ford (1.04)

DaimlerChrysler (1.08)

Thanks,

Will