Suppose the speedometer of your car reads 90km/hr. If you are driving in the United States where the maximum speed is posted as 50 mi/hr by how much are you exceeding the speed limit.
1 km = .621371 miles
90 km = 55.923.. miles
so 90 km/h = 55.923 mph
(had that exact problem a few weeks back when I drove about 4000 km through the US with my km/h speedometer car)