speedometer stops at 70 mph even if you are doing 90mph on 1999 Oldsmobile Aurora

as you decelerate under 70 mph it works fine

Asked by for the 1999 Oldsmobile Aurora
Since the limit, in most cases, is 55 mph, what's the problem? Are there limits over 70 mph in NH.? (I don't know.) How do you know that you were ever going 90 ??