My concern would be for the DYNAMIC forces at work...not just the under 180# weight of the granite slab.
Think
p = mv
which measures the momentum of a moving object
Without actually coming up with numbers, consider the momentum of a 180 lb. granite table strapped to a Sentra at highway speeds when suddenly, you have to brake hard or make an extremely evasive maneuver!!
You now have a dangerously heavy granite Frisbee launched from the top of your car!
I had my old VW loaded with about 40 lbs of apartment stuff on a roof rack clamped down overhead. I had to brake hard at one point, and the ENTIRE rack, contents and all ejected several feet in front of the car!! That was scary! Can't imagine how a granite table would fare in the same situation...