So I'm one of the few people who was bored enough to buy a car.
It's a shame, they're an interesting feature but totally wasted. I fight in turnbased, meaning that every shitty encounter me and my merc get out of the car and have to kill every single pigrat before the game will let us drive off. I'm not entirely sure what we were doing outside the car to begin with.
As I'm typing now, I've just got into an encounter with molerats, pigrats and a hermit and have spawned 20 hexes away from my car. How the hell did that happen?
But, point is, nobody uses cars. So, I suggest that so long as you're driving your outdoors is crazily high. The equivalent of 200% outdoors. And the cars themselves are a lot, LOT faster. And Gordon's Gas Station will actually refuel your car for a very small fee. Like 100 caps for a full tank.