My car runs on methane, but it's very expensive, only 20% cheaper than gas and soon it might be 1:1. Hard to store (200 bar pressure tank) and tanks have a 20 year lifespan.
Also interesting
> Compared to other hydrocarbon fuels, methane produces less carbon dioxide for each unit of heat released. [...] methane, being the simplest hydrocarbon, produces more heat per mass unit (55.7 kJ/g) than other complex hydrocarbons.
A friend has a CNG Seat Mii, that's the same as a VW Up!
Here, if you click in "Más información" on any category, you can see a list of vehicles you can buy directly to the brand ready for CNG. The site might be out of date, as the models I see there are 2 to 5 years old.