In the past, I would have tried to fake it. I.e. I would have picked a conversation and pretended to be following by nodding along so that it didn't look like I was in my own world.
I think now, I'd maybe do that if I felt I could kinda follow along but try to just be myself. If these people are my friends then one zone-out in a car won't make them drop me as a friend. Zoning out doesn't mean being a jerk. If they say something about it you can tell them the truth: Oh yeah, sorry, I have trouble paying attention when there are multiple conversations taking place at once so I couldn't follow along in the car. It's that simple. They should be able to accept it. If they are your friends, the awesome things about being friends with you won't be cancelled out by the insignificance of you not being able to follow along multiple streams of conversations.
I think there's a difference between being stand-offish vs. having to exert an unreasonable amount of effort to socialize. You shouldn't be expected to do the latter and you don't have to put yourself in a position where you force yourself to do the latter. The people in your life should understand when you give explain that you'd have to do the latter and it's too much. Does that make sense?