This is good, but it needs a twist.
It goes like this in the trilogy.
Christine, Maximum Overdrive, Cars.
One killer car, virus spreads to all cars, humanity dead-cars rule.
— Gail Simone (@GailSimone) June 10, 2016
Clearly, if you ever play a Cars-like campaign, you need to have this be the first layer of the conspiratorial onion. Have your PCs learn this (that sentient cars killed off humanity) as the Awful Truth… and then hint to them later that there is yet another, even more awful layer. You see, what actually happened is that AI erupted in automobiles – and everything was fine! The sentient cars were chill and everything. Things were great.
And then word came of a solar flare that would kill off all higher-level animal life on Earth. But the cars would be fine. So humanity betrayed the sentient cars, and somehow transferred their own consciousness into the AIs’ ‘brains.’ And that’s what the party is set to discover, much to their horror: because the transformed cars blocked out all memory of their horrible actions.
Add a lost tribe of underground humans – or sentient cars! – to taste.
Or maybe bicycles as the enemy. Like the mimes in Shakes the Clown.