Originally Posted by Glisse

The 18" wheel did come stock on the A6, specifically the wheels you have bought are one of the OEM Audi A6 18" wheels. As I said, the 18" set will be around 9mm lower in total, but you will have around 8mm more sidewall. And I presume more sidewall is why you want to move from 19" to 18" - better ride, better wheel protection.The 18" and 19" are both within spec of the suspension, the ABS system, etc etc.

As I mentioned, the C7 A6 is made for 17, 18, 19, 20 and 21 inch wheel combinations. Audi do not change any setting with the car to accommodate different wheel sizes, as long as the wheel and tyre specs are correct to OEM. The 19" version is not the "magic number" the car was designed for.

If you have an acute concern with ground clearance, stay with what you have and get a more forgiving tyre. You could put a 235/50R18 tyre on the new wheels to give you a bit more than your current total height, and a lot more sidewall, if that is your primary concern. But that is not OEM spec and will throw out your scrub radius by 5 mm, if you used the calculator I linked for you. Probably okay, may have a slight affect on steering / braking response as it is 5mm positive on a car with front drive axles, which need to maintain an overall negative scrub radius. But obviously Audi feel 245/45R18 works best with those wheels on that car. Your call.

Originally Posted by Tanket

So, 9mm lower means 18mm total, it looks a little more than that but I might be wrong. I was thinking that Audi changed something on the suspension to keep the ground clearance the same with either option, so you think not. I have to try them, at least thanks to you I now know that they are the right wheel for the car and not an A5. I don't really want, or need, to buy any other tire as both sets are new. So how do you know so much about Audi's? Thanks for all the info.