I've found that the speed limit frequently carries over when I turn from one street to another, even if the new street has a different speed limit. It usually corrects itself when passing a speed limit sign. Of course, the car occasionally also reads speed limit signs that only apply to trucks or vehicles with trailers, and that can happen on the highway. You can always override Tesla's suggestion with the scroll wheel.
I have also noticed one spot on a highway near me where the care inexplicably slows down by 10mph even though there's no sign, so I assume it's also using some sort of speed database, whether it's from Google or not. I would think Google would be more accurate, though.