Duty is charged based on where the goods are manufactured and what the items are. Some countrys duty payable can be as high as 22% depending on what it is and where it is made.
If, when you are importing a car over 25 years old and you use the tarif code mentioned in the previous posts the Customs agent usually overlooks the duty aspect for German manufactured goods and you don't pay anything.
It all depends on the Customs Agent and what type of day they have had tho.
So if you payed 6.1% you didn't overpay but I'm betting you didn't use the tarif code from the earlier post.