It is not necessary to have an International Driving Permit (IDP) when renting a car in Germany but all rental companies recommend that you have one. They are available for about $15 at AAA and American Auto Club offices, the only only authorized issuers in the U.S. They can be purchased online but usually at highly inflated prices. The IDP supplements but does not replace your valid home driver's license which you must have to rent a car anywhere in Europe.
Explore the backroads of Germany via rental car