Look here
And to:
Low-tech Magazine: Gas Bag Vehicles
Natural Gas Bus in Zigong, Sichuan
"Fish tail" would be sufficient to store 0.7 to one cubic metre of gas by visual calculations. Or to one liter of gasoline equivalent. Should any one check this on real example ? If fish tail is useable for 60++ mpg, storing methane in it should give 10-20 miles "free" miles.
For skeptics - if adding batteries for toyota prius to get 10-20 ev miles is good, this mod could cheaper and more efficient.