I just have to ask as this is could be the possible issue. In the DIC personal Settings, are you indicating KMH insisted of the desired MPH?
The school zones comment throws me this way.
Where I live 85 miles froth Mexican Border, we have many vehicles, trucks and cars from down there just poking along and I finally figured out they were using KMH and always oblivious to the problems they were causing and constant horn honking.
You can usually see this on your Speedo Dial in the subset of the analog face.
Most cars are set to be a few miles slower than indicated so the Manufacturers are not selling vehicles showing the Speed to be lower on the dial than you are actually traveling so they are not at fault if you receive a citation and were just then taking the new vehicle home from the lot. I've always wondered if we presented a vehicle to the dealer for warranty work with 35,975 miles on the clock and be told they checked the computer and discovered we were actually closer to 37,00 miles.
When I used to take my vehicles to the AAA Speedo checks, I'd be showing 1 or 2 MPH under at 25-35 MPH and at 60+, usually 3 to 5 less, and it dropped off even more at 70-75 MPH.
Driving in Utah back in the day of the 85 MPH limit, and with GPS, I was showing that I was barely doing 78 MPH. I never figured out if this was from Digital Lag and the early days of personal GPS units.
Your Milage May Vary
With my 2010 Avalanche, I was at 35,925 miles and had to go to the dealer to get a part for another vehicle. I knew there were some TSB's to resolve and when I spoke with my MR GoodWrench Guy and explained I had 3 days of busy stuff to do and expected to be putting 75 to 100 miles onto truck. He told, "Drive Backwards".
I went ahead and let hm do the work right then. While this was going on, I perused the lot and knew they had 2 4x4 2013 Black Diamonds in stock. When the 2010 was finished, I was told by my very well known to me Mr GoodWrench Guy when he pulled me aside and said this dealer wanted my truck a whole lot more than I wanted a 2013.
It tuned out, he was not wasn't kidding.