In the United States, speed limits are given in miles per hour, or mph. In many other countries, speed limits are quoted in kilometres per hour, or kph. You can determine the conversion between the two.
Multiply the speed in mph by 1.609344km per mile. For example, 55mph is 88.51392kph. For most practical intents, multiplying by 1.6 is accurate enough.
Convert from kph to mph by dividing the speed in kph by 1.609344. For example, in many countries the highway speed limit is 90kph. This equals 55.92mph. In most cases, dividing by 1.6 is accurate enough.
Learn the various typical speed limits. If you have a European car self-imported to the U.S., you will need to know certain kph equivalents. These are 48kph for 30mph, 64kph for 40mph, 72kph for 45mph, 80kph for 50mph, 88kph for 55mph, and 105kph for 65mph.
Highway speeds permitted in Europe vary between 80kph or 90kph for secondary highways and 120kph to 130kph for super highways. This translates to 50mph to 56mph for secondary highways and 75mph to 81mph for super highways. The German autobahn has sections where speed is not limited as long as it is appropriate to weather, traffic volume, etc.