The first company I worked for in the oil patch had a fleet of Chevrolet 'heavy' half-ton pickups which ran on either gasoline or propane. Before you were issued a company truck you had to learn to fuel your own. It was great, because you could go over 500 miles on the tank of propane and still have the full gas tank to drive on. The only drawback was that you had to start on gasoline and warm the engine up when it was below -30 out, or you took a chance on ruining the engine because the propane would not vaporize. The biggest plus of the system was that the engines ran cleaner than the average carburetted engine and we were getting 150K or more out of small block Chevys (350 CI) back in the late 1970s before they needed to be torn down, when 100K was considered to be the usual lifespan of those engines.
Back in the late 60's and early 70's, damn near every farmer in Glasscock County (Tx) converted their pickups and farm trucks to run on propane, some even had converted their cars. Most would run a tank of gasoline for every 2 tanks of propane because the dealer mechanics had warned everyone to run some gasoline through to keep the piston rings from wearing too much. When we rode in the back of the pickup, the propane tank, which was mounted in the bed just behind the cab, made a great bench to sit on, especially in the summer, as the gas expanded and cooled the tank as it was used.
After the 1973 embargo and the rising cost of propane/butane due to the fact it is made from natural gas, which at the time was much scarcer than oil, most dropped the conversion of their new vehicles. Most of them went diesel.