Until you get used to metric, you'll see speed signs that say 50 km. As a fast way for you to know what that is on your speedometer, multiply by .6. If math isn't your long suit, just multiply by 6 and drop the end number.
100 kmh is roughly equal to 60 mph. 50 kmh is roughly equal to 30 mph. 30 kmh is roughly equal to 20 mph.