Can i use gas in a CNG vehicle? 2000 4.6 ford crown victoria?

Can i use gas in a CNG vehicle? 2000 4.6 ford crown victoria?

If by "gas" you mean "gasoline", then no as the stock gas tank and fuel system would've been replaced with the CNG setup. If you mean "gas" in the context of compressed natural gas, then yes, you can use it.