Germany has the same 50mph (80km/h) speed limit as almost everywhere in mainland Europe except France and Belgium although you will see some outfits with a 100km/h sticker. Subject to certain technical conditions being fulfilled, one can obtain approval to tow at 100km/h (60mph) in Germany, but the process for foreign visitors would be rather arduous and hardly worth the effort for a short visit.
However, having said that, I have not heared of anyone being booked for speeding if exceeding 80km/h by a not unreasonable amount on a delimited stretch of motorway. I normally set my cruise control at 90km/h. Just be careful to observe an 80km/h speed limit in road work areas as speed traps are frequent there.