They told us that, too, in the 60’s. We tried it. Remember the 88 kph numbers underneath the 55 mph signs? .....
They’re still there in South Dakota. They put up the dual unit signs for a few years starting about when the 55mph speed limit law ended.
The neat part is that when you switch your speedometer to metric, it seems like you’re going faster. The speedometer on my Australian car goes up to 200!