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The system uses a combination of visual data from the signs and data stored in the navigation system's database. There's a routine in the software that displays the database's value if the car hasn't "read" a sign for a predetermined period of time, plus virtual 'markers' on roads in the database that trigger SLI display updates. That way you're not staring at "---" for extended periods of time.
It's possible that the speed limit was increased but the 2013 map data doesn't have the newer 75 MPH limit.
Same with the speed limit in Austin. I know it was recently raised in Texas. And here's why: the system uses stored image recognition to display SLI data. It doesn't read the numbers. It matches entire signs against the database. 80 and 85 MPH signs don't exist in the database - yet - because they're relatively new to USA/NA. So when it passes one, it ignores it - there's no match. The 2014 navigation update should resolve this.
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