I feel like your car makes you this awesome singer that everyone wants to listen to, and then all the sudden you try to sing in public to the same song you were jamming to in the car and you suck hardcore. I don't get it. When I'm in my car I jam to anything off of Eric Church's Live CD but specifically "How About You" it gets me every time!
Buzzfeed wrote a list of songs that everyone loves to belt out in the car and I have to say, they aren't any "How About You" but still some solid jamming songs.
A few of them are... 
1) "Bohemian Rhapsody"
2) "Don't Stop Believing"
3) "Sweet Home Alabama"
Did they miss any of your favorites? Check out the rest of the list here