I've noticed that those signs seldom agree with my Garmin. I'd like for any LEOs reading this to chime in. I'm not looking for room to cheat, I just like accurate info.2 options I see. Get a GPS and go on a straight road thru the different speeds to see where it shows an issue is one. The other is to borrow one of those speed warnings that show you your speed and drive up to it and see what it sees vs what your speedo says.