Like propane, CNG does not go bad in storage, and emissions are relatively clean, reducing air and water pollution by 70 percent, according to Blue Gas Marine. As with LPG, evaporative emissions and fuel spills are nil. The company doesn’t promote CNG as a replacement for gasoline, but rather as a reserve fuel. One gallon of CNG weighs just 1.07 pounds, versus 6.1 pounds for a gallon of gasoline. That sounds good until you realize you need a much greater volume of CNG to produce the same energy as a gallon of gasoline, a unit known as a gasoline gallon equivalent (GGE).