Robert Rapier writes: "Often when I am flying, I think about the amount of fuel that the airplane is burning. Then when I am off the plane, I usually forget about it. I have heard mixed opinions on the overall efficiency of airline travel versus automobile travel, but just never got around to investigating the matter myself....."
After reviewing a bit of math he concludes that his per capita fuel economy on a recent 757 flight was 81 miles per gallon.
And then he notes, "Of course it is important to note that while the fuel economy looks pretty good, the miles traveled are very high relative to automotive transportation. I generally travel less than 5,000 miles per year with my car, so if I drive a car that gets 25 miles per gallon it would only take about 16,000 miles on an airplane to equate to an entire year's consumption in my car."
When comparing airplane and car fuel efficiency, there are a couple of allures of flying:
First, the MPG of flying is better than any car. But as Robert Rapier notes, many people fly much further than they drive. One could easily sweat the details of a car purchase--trading in a 30 MPG car for a Prius--to be green, but emit the equivalent annual CO2 savings on two round trip flights from the east coast to the west coast.
I created these graphs a couple of years ago to get a better appreciation of how distance and MPG tradeoff. (The maximum MPG in these graphs is 50.)
The second allure of flying is that the plane was going to fl;y anyway so my marginal CO2 contribution is zero. But this calculation ignores the cumulative effect of individual demand. Higher demand in period t leads to more scheduled flights in t+1. It is an interesting social dilemma where each individual can feel like he or she is flying for free, but the plane wouldn't be flying if there were no passengers. Somewhere between the average CO2 contribution and the marginal CO2 contribution is the "true value".