CNG (compressed natural gas) conversions in the U.S. are very small compared to other countries in the world, we have over 300,000,000 vehicles in the United States but only around 120,000 vehicles have been converted to CNG. The first wave of CNG conversions and CNG filling stations came about the mid 90’s when the Clean Air Act was introduced, at that time the U.S. government would give out tax incentives and rebates for vehicles converted to CNG that had been “certified” by the EPA, these were new vehicles mostly trucks and a few cars, because of the rebates there were a few vehicle manufacturers (Ford, Dodge Chevy and Honda) that produced OEM (Original Equipment Manufacturer) bi-fuel (gasoline and CNG) and dedicated (CNG only).
Eventually the mandated expired or was ignored and today the Honda Civic GX is the only OEM CNG vehicle available, it is dedicated, CNG only with a range of under 300 miles.
By setting the precedent that all CNG conversions must be certified and since you can only certify new vehicles added to that all the legal hoops a company must jump through to sell conversions most of the EPA certified system suppliers have stopped selling in the United States, these are suppliers inside and outside of the U.S.. Anyone today who wants to convert to CNG has 3 problems, 1) no information is available about CNG conversions in general, 2) no U.S. manufacturer of CNG systems will sell or talk to anyone in the U.S. for fear of EPA regulations and 3) all aftermarket (non-EPA Certified) systems used to convert vehicles in the U.S. must be purchased outside the U.S..
The EPA may not be the biggest stumbling block for converting to CNG but they continue to cause the most concern to would be converters.