[Big Dave]

CNG is certainly beginning to make more and more economical sense.

Around here I can buy it in filling stations for about $2.50/gge. If I compress it (with a Phill) at home it costs $1.30/gge.

CNG also has another economy advantage: CNG gives you an honest 120 motor octane. It won't knock or ping regardless of compression ratio or spark advance. More compression general yields better efficiency which means better MPG. Practical limit is 13.5:1 compression ratio. Above that pumping losses exceed the thermodynamic gain.

Same with spark advance. In general, more spark advance = more efficiency, but at some point the limit is when the spark advance begins to break conn rods.

CNG's octane rating is so astronomical that even a 21:1 compression ration diesel will not compression ignite it. Natural gas is the one hydrocarbon I can think of that defeats a diesel. "Dual-fuel" engines use a little shot of diesel to compression-ignite and blow-torch the natural gas into combustion.