My car runs on methane - that is, natural gas or what here in Sweden is called Biogas. The biogas is manufactured by letting garbage putrify, more or less. This is good in several ways: The methane is in itself a very powerful greenhouse gas, and with a "normal" garbage dump, it would just leak into the atmosphere. Now, it is collected and used for energy, and while it does burn into CO2, it is a lot less than the equivalent for the methane itself. So the biogas is more or less climate emission negative (the net effect is less of an effect on the climate than if it hadn't been used). Burning natural gas in the engine is only slightly better than using ordinary gasoline/petroleum/whatever it is called in english (Here, it is "Bensin").
We don't do the liquefied thing (LNG), though. The refill network here in Sweden (and parts of the rest of at least northwestern europe) uses high pressure compression instead. Apparentlym it sometimes confuses people who drive on LPG (Liquefied Petroleum Gas, I think), who try to cobble together adapters to refill their cars from the CNG pumps. That is usually a bad idea. The LPG is stored in a tank under 20 atmospheres pressure. The CNG is compressed to about 230-240 atmospheres. There has been a few incidents of people actually managing to connect a LPG car to a CNG pump. That is usually followed by a big bang, a rain of metal, and the slight job of collecting the car in parts from around the nearest block or so...
