[thackney]

If every vehicle used propane...

I don't think there is ever any chance of that. More of a trimming of the peak than replacing the entire gasoline supply.

[Dusty Road]

Back in the 70’s we all were running propane and gas. You could sure tell the power difference when you switched over.

[norwaypinesavage]

Even as a "peak trimmer", the supply of propane would be severely limited. Propane has a huge advantage over natural gas: Propane liquefies at modest pressures. This means that the container of propane will weigh less than the propane it contains. Natural gas will only liquefy at extremely high pressure, or at cryogenic temperatures.

In either form, natural gas is expensive to contain and distribute in containers. Note that oversea shipping of natural gas involves cryogenic containment on the ships. Since the quantity of available propane as a percentage of available natural gas is very small, it's not realistic to consider propane as anything but a very small percentage of transportation energy. Anything else would make propane quite expensive for home heating and barbeque grills.