Just for fun I did some back-of-the-envelope calculations comparing the use compressed natural gas (CNG) to gasoline for fuel in an automobile. I assumed the auto gets 20 miles per gallon on gasoline, and that the efficiency of the auto is identical for either fuel. (That is probably a reasonable assumption of the same engine is used, but an engine designed specifically for CNG might be better.) Based on that, the cost for the gasoline fuel would be 17.5 cents/mile if gasoline is $3.50 per gallon and 15 cents per mile if gasoline is $3.00 per gallon. For CNG the cost is 4.56 cents per mile at the current price of $8/mcf and 6.84 cents per mile at the recent high price of $12/mcf. So, it appears that CNG is less expensive than gasoline until gasoline gets done to $1 per gallon, which seems unlikely. If one is concerned about the amount of CO2 produced, I didn't do a detailed analysis. But the weight of fuel used is a good indicator. For gasoline the mileage per pound of fuel is 3.07 miles/lb. For CNG the weight required is 4.55 miles per lb. So, less CO2 would be produced by CNG.