Whether an International Driving Permit is required in the USA depends mainly on the state.
Kentucky, Georgia and Florida are considered low tolerance states for foreign driving licences. But even in other countries, you cannot rely on your German document in every case.
Most on-site car rental companies are relaxed if you do not have an International Driving Permit. However, there are also exceptions that do not want to recognise European driving licences.
It becomes even more difficult in the case of a police check: US police officers are usually very sensitive to foreign documents without an English translation.
The situation is similar in Canada: The International Driving Permit is not compulsory, but it makes communication with car rental companies and authorities easier.
We recommend an International Driving Permit for all travel to Canada and the USA. With this document in your luggage, you can overcome bureaucratic hurdles faster and easier.