Wolfgang,
One more thing! Since my speedster is a widebody, I have 225x17 tires on 7" polished rims in front and 265x17 on 9" in the rear.
I usually take these off for the winter....while the car is in hibernation 
and put instead old 185x15 on 5" steel wheels on...just to be kind to the fat rubbers ...but able to move the car around if necessary. The car looks like an ugly duckling that way....but it serves the purpose.
I took the car to get FIRST inspection after it arrived from the states on those old steel wheels !! Just to not freak them out at TÜV right away.
Later I went back to them...with a bunch of additional documentation for the wheels (i.e. US patent number, entry in the patent registry, engineering expertise, load capacity and mfg specs) and got a "specialty" permit to have them mounted and operate the car with them! Its an additional charge of about $85 and is written in your vehicle registration docs. Once done, you´re ok.
Of course its a hassle, but especially with wheels and such, they are extremely strict in Germany. The main problem is....if you drive on ´em without having them written in the car´s registration docs.....and anything happens....like any small accident and God forbid it involves another car or person (we all know that the devil is in the details)....the insurance company will NOT pay a cent...not for your car and definitely not foe anyone else involved!! Their excuse will be, that the car had no official TÜV to be driven with those wheels and tires and thus does not conform with standard safety regulations!!
So beware!!