I think the majority of Americans would say "Which car am I driving in?" However, something makes me think the majority of Britons might actually say "In which car am I driving?" Am I the only one thinking this?
The older and more educated the Brit, the more likely that is. I would sometimes say that, but the majority of Britons today would say "wot car are we drivin', eh lads?", or ask in Urdu.
You probably aren't the only one thinking it, but it isn't true. Sentence-ending prepositions of this form are quite common throughout the English-speaking world.