Hot Rod Dad!
There are conversions available that allow switching 'on the fly' from gasoline to propane and back.
I ran a Crown Victoria on CNG/gasoline (compressed natural gas) and the switch back and forth was almost seamless. Had about 80,000 miles on that car before moving to an area where the CNG wasn't available. I commuted to work nearly everyday and the cost of natural gas was way less than gasoline. At the time, the trip on gasoline cost me just over $18. On the CNG it was under $8! They had a device available called a VRA (vehicle refuelling appliance) that could be hooked up to your house line and then pump up the tanks in the car. Took a long time to fill them up to 3000lb pressure. The cost of running the appliance was akin to operating a clothes dryer for a few hours at a time.
Although the economies of operating on CNG were great there was the drawback of weight. The tanks were big and heavy. They were certified at 6000lbs pressure but you could only fill to 3000lbs. I had to put cargo springs on the car and of course the brakes took a beating because of the extra weight.
Cheers, George
