I took my car off the boat, so I took the stock 18's somewhat reluctantly. I almost immediately added 20mm/15mm spacers to the car (running ET 27 rear and 32 front) per the recommendation of that encyclopedic AZ article on the topic.

The other day I installed a set of stock 19's (peelers) that I bought from an AZ member who upgraded. After a couple of days of typical (read: slow on the highway, reckless in the city) driving, I have some commentary on the whole process:

- Center caps weren't part of the deal, so I went to my local dealer (ECS was out of stock) and left with part #4B0 601 170 A. These turned out to be 1mm too small; they went on different sets of peelers, either for the Q5 (20") or the S5...not sure. The proper part number for the S4 version is 8T0 601 170 A. The parts counter guys couldn't even tell the difference at a glance.

- I purchased 8mm and 6mm spacers for this setup, only to find that there wasn't enough hub left to "properly" mount the wheels. I left them off for the time being...

- ...which might turn out to be a while. I had attributed a "shudder" during low-speed, edge of turning circle cornering to AWD and sort of got used to it. With the stock setup, it's gone. Even some funkiness in high-speed cornering has disappeared leading me to one of two conclusions: either (1) the extra inch of wheel makes a huge difference or (2) 40mm of additional track threw things off enough to make for a sub-optimal driving experience. It feels like I have new, better handling car. I came from a string of 5 or 6 BMWs (all but one RWD) and had been attributing the differences to the brand; I've since changed my mind and now blame it on having to modify the suspension so that the wheel wells didn't look like a '99 camaro (sunk into the wheel well and slightly small).

In short, my experience tells me that the 19x8.5, 255/35 setup is near-ideal...

 

Are you sure perhaps one of the 18"s isn't bent or out of balance? Even a bubble in the tire can cause that feeling you describe.

There's no reason why you should experience shudder at any speed with 18"s. That's what I currently have and have zero issues or complaints.

 

I have 255/40/18's ET35 and they do not shudder at all. Not sure from where your difficulties originate from.

Bruce

 

Well, at one point I had it in the shop for something and had a ride-along. The shudder I get is not from a bent wheel (I know what that feels like) but likely from power steering pump, which (according to the tech) works harder at the locks and I suppose makes it into the steering feel. I'm not sure how that works, but that was the explanation I got.

After my commute today, I can say with even more certainty that this setup is far, far superior to the 18's with a wide track.

 

You found out the hard way why spacers can be a bad idea......