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Propane as an enhancement doesn't make much sense for a gasser, while there were some folks resorting to it on Diesels. As propane is outlawed as a motor fuel in my country, even though it's allowed for forklifts, I have already seen some big rigs using CNG enhancement for both mileage and emissions, decreasing the usage of DEF noticeably too.
When it comes to spark-ignition, propane is not so bad at all, but I'd rather take a look at some of those newer liquid-phase injection systems, instead of the vapour-phase setup which is still more usual. But neither propane or CNG are worth as MPG boosters on a gasser, yet under some circumstances while being used instead of gasoline at least CNG may be cost-effective. Too many people resort to an excessively lean AFR with CNG in my country, as it has anti-knock properties even better than ethanol, to the extent such an excessively lean AFR could eventually lead to an overheating, not to mention a performance loss. A proper AFR, even though the cost-per-mile advantage of CNG would go a little down, wouldn't cause much of a performance loss and decrease the likelihood of an overheating.
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