Megabus.com adds Philadelphia-D.C. line, from the Daily Pennsylvanian.
We learn from a Megabus spokesperson that their vehicles use "less than a pint of fuel per passenger mile".
For those of you who don't have the misfortune of knowing this, there are eight pints in a gallon. So these busses get better than eight passenger-miles to the gallon!
Since most cars in the US get at least 20 miles or more to the gallon, this is really nothing to be proud of.
(I'm guessing that busses are actually more fuel-efficient than cars, at least if they run sufficiently full.)