Originally Posted by Faceofboe
Gary, thank you. I have to say that the TireRack was great. I received all the tires, mounted and balanced within days. They even put the BMW center caps on! I wish I had ordered from them a long time ago. The fitting is perfect, right down to the TPM sensors. I'm just confused about why the mechanic was so odd about having smaller tires.
I assume you are military or otherwise under the SOFA (the ordering from Tirerack was a dead giveaway.)
Accordingly, your car does not need to meet the technical requirements in the Fahrzeugbrief for your car (assuming it is Euro spec - you wouldn't have that document if it is US spec.)
The Fahrzeugbrief (the German equivalent of a US title) spells out not only things like tire size but in some cases even approved manufacturers. Any deviation (aftermarket wheels for example) must be tested for that application and an entry made in the document if approved. Some manufacturers will get pre-approvals for certain applications so that no separate entry needs to be made - but the manufacturer's approval document needs to be kept with the car.
It is quite possible that a 16" wheel might require a supplemental authorization if
the 328 doesn't come with 16" wheels fitted as standard in Germany. However, the 328 comes fitted with 205/55 R 16 H tires, so your Opel mechanic was wrong.
Long story short - you're perfectly fine on the 16's. The smaller tires will give you superior winter performance and would be the appropriate size for that car in Germany and in the US.