Speed on the AUTOBAHN is a matter of the suspension mods and oil pan volume plus proper ignition system to hold for higher speeds.
We have some pretty modified cars that are able to maintain (240km/h), about 140mph over a longer period.
However most unmodified rather cruise at speeds about 70-90 mpgh as the Mustang usually tends to wander due to the carved in tar and sloppy steering linkage. 100-120 mph is at least the range, everybody has tried, but came back with white knuckles from the ride of his life in unmodified status.