A dedicated natural-gas vehicle, like the Civic Natural Gas, runs exclusively on clean-burning natural gas. This guarantees 100% alternative-fuel use. Some other natural-gas vehicles use a "bi-fuel" system that doesn't offer the same economic and low-emissions benefits that a dedicated system offers. A bi-fuel vehicle has two different fuel tanks—one containing traditional gasoline, and the other utilizing an alternative fuel such as natural gas.
•How many gallons of gas would you save annually if you bought one of these vehicles? Weight-saving technologies, improved aerodynamics and engine modifications result in a new Civic Natural Gas with an impressive EPA rating of 27/38/31 miles per gallon (gasoline-gallon equivalent). The recent engine and transmission changes have another upside: an increased range of over 10 percent, up to 190 miles.  27 city/38 hwy/31 combined miles per gallon (gasoline-gallon equivalent). Based on 2012 EPA mileage estimates. Use for comparison purposes only. Do not compare to models before 2008. Your actual mileage will vary depending on how you drive and maintain your vehicle. Performance Features
http://automobiles.honda.com/civic-natural-gas/performance.aspx •Why do you think there are not more of these vehicles in use? For one they cost to much. The price for one of these are $26,305