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.)