WIkipedia has an article on .'gassification'; In WWII, the gasogene was a French truck motor that ran directly on coal(among other things). Think Model A ford style trucks prowling the wastelands, I smell fan0-fic potential brewing...

Coal is assumed to be all used up.

Any post apocolypic energy sources will be burning pretty much whatever is available, rags, books, etc. Probably would then move on to alcohol based fuels (ethanol)

But the fallout bibles reveal steam powered trucks were planned for FO1.

It's still possible to restore vehicles in the Fallout universe. In Fallout 2, you could restore a Highwayman that ran on Microfusion Cells.