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Autopilot Speed Limit Anomaly

Why does Autopilot want to break the speed limit all the time? When entering a section of road where the speed limit is lower than the one you have been driving down, autopilot will limit the speed but not to the actual speed limit or even the limit plus what you told the car you want it to drive at, instead it chooses 5 miles per hour above the posted limit. This is NOT adjustable.
You can set a number of miles per hour above the limit that you want autopilot to adopt when you turn it on but it ignores this when changing the speed limit due to posted limit changes.
What has this got to do with the Autopilot automatically reducing your speed? The settings here have no affect on what the new speed that Autopilot adopts is. If I am driving at 45MPH on autopilot and come to an area with a 35MPH speed limit Autopilot will change to 40MPH. Not 35MPH, 40MPH. 5PMH above the speed limit. Nothing that I have found, or that support has been able to identify, will change this behaviour.


Beyond Driven
Nov 10, 2014
The Americas
What's even more fun is when the AP2 speed limit data is wrong. So there you are, moseying along at 45mph (TACC @ 50mph presuming you have a +5mph offset), the speed limit sign you just passed sez 35mph, and now you're breaking the law by 15mph in a 35mph zone. Good times.

At least with an AP1 car, the 35mph sign is read correctly, which reduces speed to 40mph including the offset, unless in the rare case that there is a big truck tweenst yer car's camera and the new sign.


Active Member
Apr 5, 2012
An even worse example...

The "new" speed limit database distributed last year has many cases with speed limits in the database that are too high, too low or missing.

If you let TACC automatically set the speed using that database, you could run into a situation where AutoPilot will drive considerably above the posted speed limit.

Near our house, we have a 4 lane divided surface street (with a narrow lake separating the lanes). On one side, AP thinks the speed limit is 55 MPH and tries to set AP to 60 MPH, even though the posted speed limit is 40 MPH. On the other side, there's no speed limit data at all, which is probably better than having the car think the speed limit is +15 MPH...

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