Depends on the size of the tree. When working as a firefighter/EMT, I saw some pretty big trees chopped off by vehicles and the passengers with surprisingly little damage.
Also, the crumple zones of newer cars are outrageously effective at absorbing impact, so you'll need to be travelling very fast to overcome that.
And as @maka says, it's a pretty awful way to go, when it is fatal. It is rarely so instantaneous as Hollywood would have you believe.