Cng
So I looked into this a couple of months ago for my 2005 ranger and heres the deal.
The CNG is cheaper, about half the price of gas, and cleaner burning. However, pound for pound it has less power. Now, the CNG is actually ideal for an internal combustion engine because it is already gaseous, but the retro fit is expensive. Most often the CNG fords I see are cabs and busses that are fleet vehicles expecting 1,000,000 miles of service, which will eventually pay for the conversion through fuel cost savings.
The conversion involves new injectors, fuel line, fittings, and most expensive is the tank. The company I found on the web wanted to charge between 5,000 to 8,000 dollars! Depending on tank size. The reason is that unlike LP propane, CNG is a highly compressed gas, like 5,000 psi in the tank! You can fill up at your house though from 60 psi line pressure using a special compressor. But remember that those compressors use electricity and money to operate as well.
in the end, it won't be efficient (cost effective) for the average joe to run CNG until the car companies build them that way, though some states have tax credits for a couple of thousand dollars to offset the cost.