I totally agree with you Puritan that it would be hell driving four adults 500 miles in a Ford Focus. I also agree with Touch&Go that it might not be entirely easy to find 4 adults that want to make the trip at the same time.
So why don't we put 2 adults in 148/2=74 Ford Focus' then? Each Focus will by now surely manage its 50mpg on the open road. If we share those 736 gallons of fuel between the 74 cars they get just under 10 Gallons each. The cars do just over 50mpg over the 500 miles and end up using their 10 Gallons of fuel with none to spare.
My point is if you try to use this argument of efficiency against a "Greenie" who is trying to add tax to aviation then they can say that with 2 adults per car, the car is still as economical as a FULL 737 and that by increasing the number of passengers in the car higher efficiencies can be achieved. This is not to mention the fact that not every 737 is full and that other cars exist with higher fuel efficiencies...
Let us face facts. We don't fly London to Edinburgh to reduce fuel consumption. We fly because it is quicker and safer and more comfortable due to shorter time sat in a seat.
Be careful to ensure your arguments cannot be turned around and used against you.