So last year I started writing a post-apocalyptic story, and then realized I had no idea what the world was really like, and quite a few things just didn't seem to make sense.

One of the biggest that jumped out at me was the abundance of running automobiles tooling around in my story. Which got me thinking: Could such a thing really happen?

To set the stage, the short version is that in the near future Earth was the target in a cosmic shooting gallery of cataclysmic proportions: While no single blast even matched, say, the Tunguska event, there were so many across a period of many days (weeks, even) that virtually the entire surface of the planet was devastated, and every major city (and many minor ones) was pulverized into oblivion. (What no one in this world (yet) realizes, though, is that this was a targeted attack intended to wipe out civilization, but not necessarily exterminate the planet.) Cue the inevitable collapse of every recognizable society, and fast forward a few years to the scrappy survivors of the human race having banded together into communities of virtually every description scattered across the ruined wasteland of their planet.

Now, certainly, any vehicle that survived this apocalypse, and could handle the terrain of this new world, would be usable for a certain period of time afterward, certainly at least until it ran out of fuel. And no doubt there would be opportunity to scavenge some fuel here and there, but with essentially no more oil rigs, let alone refineries or the infrastructure to distribute these products, certainly fuel wouldn't last long, would it? And of course there's the myriad of other fluids that have to be periodically topped off or even replaced. Maintenance would be another difficulty -- spare parts might be easier to come across, and for much longer, but eventually you'd break down somewhere without access to what you need.

So really, I suppose the question is: Given the utter destruction of modern society, how long could one expect motorized vehicles to continue to be usable by the survivors?