When you use your German credit card within the Euro-zone, there is usually no fee for your foreign purchases. The Euro amount appears on your credit card balance as it would in Germany. However, when you travel abroad all credit card providers have to convert the local currency to Euro in order to put it on your credit card bill.
The three largest credit card providers are VISA, Mastercard, and American Express. These credit card brands convert local currencies according to their internal currency exchange rate. The bank that issued your card may then add an additional foreign transaction fee of 1-2%. You can avoid this foreign transaction fee by simply choosing one of the many banks that do not charge you for a transaction that is not in Euro, for example N26, Advanzia Bank, or DKB.
The question is: If your bank does not charge you a fee for foreign transactions, what is the difference in exchange rate between Mastercard, VISA, and American Express? We wanted to find out which credit card brand has the most favourable exchange rate and decided to look at a sample time period. We checked the credit card exchange rates for U.S. Dollar to Euro for each day of November 2018 in order to be able to compare VISA, Mastercard and Amex. See the graph below for their daily rates. We compared all three rates to the official stock exchange currency exchange rate (EURUSD=X).