Greyhound gets some competition from Megabus.com
Buses, on average, get low passenger miles per gallon in the U.S., because they stop often and don’t use most of their capacity. Coach buses — providing prebooked travel between cities — don’t suffer from these limitations.
Megabus.com, a new niche player in this market, provides cheap, comfortable travel between nearby cities with travel time comparable to driving or taking commuter airlines (in a very small portion of the U.S.). Efficiency is 184 passenger miles per gallon — without using hybrid buses or using any particular efficiency technology. They just use yield management ticket booking, where the earlier you book a ticket (relative to other passengers) the less you pay for your seat. (Thanks, Jordan Hayes for the tip.)