One thing all Thanksgiving travelers should be thankful for is how many people carpooled yesterday. I don't have any real data on this, but I was in a five-person carpool, and I saw a lot of other people carpooling. This cuts way down on traffic and pollution.
Say you're in an old SUV that gets 18 miles per gallon. With five people in it, that's 90 passenger miles per gallon (5 passengers * 18 miles / gallon). Another way of looking at that is .011 gallons per passenger mile. That's right, the five-person carpool in the old SUV uses less gas than a solo driver in a Prius, although a five-person carpool in a Prius would be at about 250 passenger miles per gallon (.004 gallons per passenger mile).
What's the best way to increase your car's fuel economy? Carpool.