This question came through the Suggestion Box, and I found it only right to respond to such a well thought out inquiry. 

Soooo, the answer is 10.5.

 

If you have four people in back lay on top of each other, and four people in front do the same, you can then cram two people into the trunk, and, if there is a sunroof, you can have a person standing on top of the four people in the front so that he/she is half in the car and half outside of the car.

 

I would encourage you to give this a try, you may be able to prove me wrong.