[SatanicMechanic]

Auto Propane is usually a few cents higher than regular gasoline. When I was in high school, I used to drive a propane powered truck for a rental company. The company sold propane, so when we filled up the truck, it was charged as heating propane. Heating propane is cheaper because it does not have the state and federal fuel taxes.
If you convert over a vehicle to propane, rent or buy a very large tank, tell the propane people that is for a water heater and refuel your vehicle at your house.

Sometime I will tell you guys about the propane powered pickup my friend bought and how he used to steal proane from neighborhood BBQ's.