To be entirely fair, the speed limits they have entered into their database are often incorrect. For an egregious example, I was on a state highway in Illinois last year (I think highway 16 or something like that) and Google Maps said the speed limit was 16MPH despite the clearly posted 55MPH speed limit, so obviously whatever person or tool they had reading the signs misunderstood the highway signs as speed limit signs

There's also plenty of local roads where Google Maps has the wrong speed limit. Some have very few if any speed limit signs posted so they clearly just have a guess entered into Google Maps ignoring the state/local laws that make it easy to guess the correct speed limit (federal highways are 65MPH, state and county highways are 55MPH, local highways/farm roads with no posted limit are 45MPH, and 25MPH within city limits if no other posted limit), others Google Maps just simply has the wrong limit entered despite it being clearly posted for no clear reason