A CNG conversion kit adapts a normal car engine to run on compressed natural gas. Cars converted to run on CNG can freely switch between CNG and regular gasoline or diesel. A CNG kit typically includes a gas tank, a fuel regulator, an additional set of fuel injectors and a custom wiring harness. The Clean Air Act generally prohibits fuel system modification, so a qualified fitter must install the CNG kit.
The main advantage of CNG conversion is cost. As of 2015, CNG fuel is much cheaper than gasoline at 79 cents to $1.50 per gallon equivalent of fuel. That cost saving is offset against the initial cost of purchasing and installing the kit, so CNG conversions are most suited to drivers with high annual mileages. Some states provide tax credits or rebates for CNG conversion, lowering the up-front cost. CNG powered vehicles are also permitted to use High Occupancy Vehicle lanes in many states.
Availability of fuel is a significant factor in the decision to fit a CNG kit. CNG is only available at 876 filling stations. CNG compressors that fill a car tank from mains gas are available but at a cost of several thousand dollars they are generally only suitable for fleet users. The CNG tank often occupies a significant amount of trunk space, which some drivers may find inconvenient.