Comparing Driving and Flying

Updated — it turns out EPA provides GHG emissions for cars!
Second update — two examples added at end.

In the previous post, the greenhouse gas cost per airplane mile is calculated at 2 – 4 times the cost of the fuel alone, anywhere from 1 – 1.9 pounds carbon dioxide equivalent per mile, a little more for people who don’t walk to the airport. How do cars compare?

Go to EPA to find GHG emissions for cars – you may want to divide their fuel economy and multiply by your own FE (typically 20 – 30% lower than EPA indicates). You can add the cost of manufacturing the car, see examples below*.

In the particular examples chosen, the Toyota Prius (assume 45 mpg) pollutes 0.62 pound carbon dioxide equivalent/mile, and the Dodge Durango (12 mpg) 2.1 pounds.

It appears that the airplane is worse than the Prius and a better choice than the Durango, at least for one person. Although the fuel economy of airplanes is actually about the same as a Prius, airplanes deposit water vapor, etc high in the atmosphere, so create about 2 – 4 times as much damage as would be expected from the amount of oil used.

However, the typical person makes choices based on time rather than distance: both commutes and vacations are determined by travel time. Many opt for some combination of more trips and longer trips. (Ditto for convenience cooking equipment – fancier meals, not reduced cooking time. Computers for easier word processing – nicer layouts, not faster turnaround.)

What figures into your decision on whether to fly or take slower transportation? Is the environment a part of your calculation?

*Separate GHG costs into two portions, the cost of manufacturing the car and the cost of using the fuel.

First, the car. The US Department of Transportation says that the average car lasts 125,000 miles.

Find the weight of your car. On the Road in 2020 (pdf) provides GHG costs of material: assuming that your car is made from mostly recycled materials and will be recycled in turn, multiply the weight in pounds or tons by 2.5 to get the amount of carbon dioxide released in manufacture in pounds or tons (note: if your car is extra light because it’s heavy on the aluminum, the GHG emissions for manufacture are higher per pound, sigh).

A Prius weighs 2,890 pounds, a Durango 6,600 pounds. Multiply by 2.5 –the Prius manufacture GHG cost is 7,200 pounds carbon dioxide equivalent, the Durango is 17,000 pounds. Assuming that the car lasts 125,000 miles, this comes to 0.058 pounds/mile for the Prius, 0.14 pound/mile for the Durango.

EPA says the Durango gets 15 mpg, emitting 12.0 tons GHG in 15,000 miles, so most drivers get 12 mpg or worse. EPA says the Prius gets 55 mpg, emitting 3.4 tons in 15,000 miles, so most drivers get 44 mpg or worse (actually, Prius drivers tend to drive more efficiently, so perhaps the Durango real driver penalty should be higher).

Divide by 0.8 to get more realistic driver emissions, and then divide by 15,000 miles and multiply by 2,000 pounds to get the per mile emissions: 2 pounds carbon dioxide equivalent/mile for the Durango, and the Prius 0.57 pounds. The greenhouse gas cost of manufacturing the Prius adds another 10% to the fuel cost (just over 0.05 pounds), 7% more for the Durango.

You can add your two figures to get total greenhouse gas/mile. For the Prius, this will be 0.62 pounds/mile. The Durango comes in at 2.1 pounds/mile.

If you don’t want to do weight separately, add 10% to your fuel GHG emissions for a high fuel economy car, and 7% for one with low fuel economy.

If you want to pay GHG tax on your car, say $20 – $50/ton (see previous post), multiply pounds carbon dioxide equivalent/mile by the number of miles/year, and divide by 2,000 pounds in a ton to get GHG emissions in tons. Then multiply by the tax/ton. Note: this ignores GHG from other pollutants, which are relatively small unless you always drive your car short trips – your car is much more polluting cold than warm. If most of your trips are less than 5 miles, round up in calculating GHG costs.

Example 1:
Prius. You get about 44 mpg, and your total emissions, car plus fuel, are about 0.62 pounds/mile. You choose to charge yourself a $35/ton tax. You drive 8,000 miles/year.

GHG emissions = 12,000 miles * 0.62 pounds CO2e/mile = 7,500 pounds carbon dioxide equivalent

Tax = $35/ton * 1 ton/2,000 pounds * 7,500 pounds = $130. This doesn’t add much to your yearly costs. At $3/gallon, you’re paying $800 for gasoline. Donate the money to improving your insulation, or someone else’s, and ask your legislator to add this tax to everyone.

More expensive example:
Durango. You get 12 miles/gallon, and your total emissions, car plus fuel, are 2.1 pounds carbon dioxide equivalent/mile. You drive 12,000 miles/year, and charge yourself $50/ton.

GHG emissions = 12,000 miles * 2.1 pounds CO2e/mile = 25,000 pounds carbon dioxide equivalent

Tax = $50/ton * 1 ton/2,000 pounds * 25,000 pounds = $630. This adds more to your yearly costs, but at $3/gallon, you’re paying $3,000 for the gasoline. Donate the money to funding a new car, and ask your legislator to add this tax to everyone.

One Response to “Comparing Driving and Flying”

  1. catharine Lucas says:

    This is exactly what I’ve wanted to know, but I wish you’d go a step further for this non-math-minded person. As a Prius owner who frequently drives less than 5 miles/trip (with occasional longer trips where my mileage is so good I don’t refill my 12 gallon tank in over 700 miles), I can see from your figures that I produce 0.62 lbs GHG/mile. To adjust for weight of car, you suggest I add 10% to my fuel GHG — Does this mean I produce 0.72CO2/mi
    or am I adding 10% of 0.62 = 0.68CO2/mi ?

    Please carry out your example, for me, so I can see how it goes:
    E.G., Let’s say I want to pay GHG tax on my car, taking a median tax of $35/ton. Say I drive about 5,000mi/year. First, you say, multiply pounds carbon dioxide equivalent/mile by the number of miles/year,”: This would be 0.72CO2/mi, taking the highest estimate
    X 5,000 mi = 3600 total CO2 emissions (?)

    Then I divide by 2,000lbs to convert to tons: So I emit 1.8 tons CO2.
    Since I take lots of short trips, and want to round up, I could call this 2tons, which also makes the math easier!
    Finally, I multiply by the carbon tax I’ve chosen ($35/ton) and get $70 a year that I should set aside for funding reductions in GHGs to fight global warming. (I don’t know yet how best to do that!)

    This is a small amount of money. Should I be thinking of how to get policy makers to impose this tax asap, and what should the funds thus generated be used for?