Using tyres from the USA is only allowed in Germany when they have an EU or ECE label (proof of design certification), informing about the load capacity and the maximum speed.
An example and what it means: The tyre label is "P205/70R15 94H"
"P" simply means "passenger car"
"205" is the tyre width
"/70" is the relation of tyre height and width
"R" stands for “radial“ design
"15" is the rim diameter
"94" stands for the load capacity and “H” for the permitted maximum speed (incl. tolerance).
If the tyre has an additional "M+S", the vehicle’s design related maximum speed can be above the one of the tyre.
A tyre with "P185/75R15 M/S", on the other hand, is not permitted and needs to be replaced before the vehicle gets registered in Germany.