I think this is a fine thread and a good topic given how often ppl tout fuel mileage as a reason to ride a motorized bicycle...
...except as others have mentioned propane conversions loose a LOT of power (even 20% is a lot if you've not much to begin with) & it stinks... literally (it's an additive, but it's still nasty). Hydrogen is simply still too complex to obtain.
I want a diesel.
Not even the hoity-toity bio diesel - jus' a good old fashioned thumper to put on me bike gdangit o.o There's just too much torque to ignore