at one time around here after comparing energy density and cost and fuel economy propane was noticeably cheaper even after conversion costs for gas engines if you put 10K miles or more/year on the vehicle. Some of my co workers who had 50+ mile round trips each day did the conversion and were happy with it. You still had to pay fuel tax on the propane. But then propane got more expensive so the advantage disappeared,
For a while folks with diesel trucks were doing a conversion where propane was injected into the manifold and diesel injected into the cylinders again, for cost advantage along with longer range (but with a propane tank in the back)
Propane can tolerate a much higher compression ratio than gas before detonation, so like alcohol, you can gain power IF the engine is designed for the higher compression and can take advantage of ignition timing. That gains power and economy.
Lots of stationary power generators and big backup generators run on propane, no worry about fuel going bad and the burn so clean oil life is much longer and they are easier to start.
Then there are dual or tri fuel portable generators. My honda 2200 is modified to run on gas, propane or NG. Propane or NG does have less power, but if I need it at home if the power goes out, running on NG give an infinite run time, no worry about how much gasoline I have. Run it on gas when camping.
