You could do some math. The elimination time on a mountain stage is up to 118% of the winner's time.

So with a 5 hour stage time, the autobus needs to be in within about 50 minutes of the stage winners time. So on stage 17 if you started L'Alpe say 15 minutes down, you'd have a cushion of about 30-35 minutes. Which means you'd have to climb at about 60% of the leader's speed.

Based upon personal observation, I'd guess around 8mph to be typical for the Autobus on the big climbs. Of course it varies depending on the situation.

They pretty much have it to a science and go only as fast as they have to. Also if the bus has enough passengers, they can finish outside the time limit, and still not be eliminated.