Chartjunk: U-Haul CO2
Posted: January 7th, 2009 | Author: admin | Filed under: environment | Tags: chartjunk | 2 Comments »Wow! Talk about chartjunk!
This chart shows that on a 1,000 mile trip, a U-Haul truck running on gasoline will emit 2,774 lbs of carbon dioxide, while some other truck running on diesel fuel will emit 2,798 lbs. On this trip, the U-Haul will emit 24 lbs, or a whopping 0.8%, less carbon dioxide! Aside from questioning the comparison of a gasoline truck to a diesel truck, this difference is so tiny (less than one percent) that its meaning is quickly swallowed up by any other variables in the system (age of truck, miles per tank, driving habits, fuel cost, total energy efficiency, weather). The glaring offense is the fact that the other truck’s dark cloud of smoke appears to have nearly three times greater area than the lighter U-Haul cloud. So, visually, the chart implies that U-Haul emits 60% less, while the dubious numbers show that the difference is less than 1%; this is a visual distortion of extreme proportions. Mysteriously, a trip of 999 miles or 1,001 miles yields a difference of only 23 lbs.
Now let’s zoom out for a reality check. Does a truck really emit 2,800 pounds of carbon dioxide on a 1,000 mile trip? 2,800 pounds seems high. A brief search points to the EIA emission coefficients, which suggest that burning a gallon of diesel fuel emits 22.4 lbs of CO2. Assuming 10 mpg on a 1,000 mile trip yields 100 gallons burned. Apply the coefficient and you get 2,240 lbs of CO2. So, that number is close enough. As strange as it seems, mixing a gallon of fuel (weighs about 6 lbs) with oxygen, and then burning the mixture does produce about 20 lbs of CO2. This is because each carbon atom freed from a fuel molecule hooks up with two oxygen atoms from the atmosphere to form a CO2 molecule. The hydrogen in the fuel also hooks up with oxygen to form water. So my guess is that the math could look like this: 6 lbs of fuel + 24 lbs of oxygen = 22 lbs of CO2 + 8 lbs of water.
Check out the excellent data attribution line — “All numbers are estimates based on averages from various reliable sources.”
Not only is this bad presentation of data, the comparison U-Haul uses between gasoline and diesel is invalid. While diesel fuel does have higher carbon density than gasoline, diesel engines are substantially more efficient because they can tolerate higher compression ratios. You cannot compare a diesel and gasoline truck that both get 10 mpg, because the diesel will do more useful work. For a given amount of cargo and a given route, a diesel truck will use less fuel and emit less CO2 than a gasoline truck. This is bordering on fraudulent presentation of data, and appears to violate the Truth in Advertising Act.
Not only is this bad presentation of data, the comparison U-Haul uses between gasoline and diesel is invalid. While diesel fuel does have higher carbon density than gasoline, diesel engines are substantially more efficient because they can tolerate higher compression ratios. You cannot compare a diesel and gasoline truck that both get 10 mpg, because the diesel will do more useful work. For a given amount of cargo and a given route, a diesel truck will use less fuel and emit less CO2 than a gasoline truck. This is bordering on fraudulent presentation of data, and appears to violate the Truth in Advertising Act.