Puritan, all points taken and agreed with.
Therefore, since most cars will do more than 23mpg, most cars are more economical than the 737 on a per passenger basis. For example:
Ford Focus 1.6 5 door will do 51.4mpg on the open road. Let us be extremely negative and guess that this will reduce to 40mpg with 4 passengers and their suitcases (Although I imagine the Ford test of economy was conducted with at least one passenger!). If we share those 736 gallons between 37 of these cars they get 19.8 Gallons each. The cars do 40mpg over the 500 miles and end up using 12.5 Gallons of their fuel which leaves 7.5 Gallons per car, or 277.5 Gallons in total unused fuel which would come in handy I'm sure.
Even so, for many reasons, I would prefer to fly 500 miles than drive 500 miles.