10.8 mph actually - I even have a 1936 sales brochure for the #11 Auto Patrol.
In fact, I have two brochures for the machines, one is a later edition than the other, and the operating weight went up 1,550 lbs between the two versions.
The early brochure says 17,000lbs operating weight, the later one says 18,550lbs operating weight.
I can see no difference in the specifications otherwise, so I guess Cat must have heavied up a few castings.