Americans have been conditioned to believe that the "real" speed limit is at least 10 mph over the posted limit.
Actually, it's the other way around. American speed-limit-setters have been told to post a speed limit that is at least 10 mph below the maximum speed people actually drive. More or less.
Or, from wikipedia:
The speed limit is commonly set at or below the 85th percentile speed (being the speed which no more than 15% of traffic is exceeding) and in the USA is typically set 8 to 12 mph (13 to 19 km/h) below that speed.