Stolen from Google;

A 16” wheel will usually also be 2” narrower than an 18” wheel, and so take a tire that is both smaller in diameter and narrower. A smaller and narrower tire has better grip and performance in snow or ice. This is both because the smaller tire cuts through snow more effectively, and because the weight of the car is pressing on a more focused set of contact patches.

Just to bump this up, anyone put 235/50/19's on 19" wheels before?

Got to be careful on that as insurance will be funny about a non-standard size tyre.

I remember passing many mercs and beemers in my old clapped out mark 1 Astra on 145 tyres in the snow years ago :)

The thing is, the laws of physics will beat you on braking and steering pretty much no matter what. I'd stick with the standard size and go with winter tyres if you need to. TBH, I've never gone with them and just drive to road conditions.

I think that really applies to summer tyres used in winter conditions, not to proper winter tyres which are designed to grip snow. ;)

Narrower winter tyres will still be 'better' in proper snow. On ice, you're screwed without studs pretty much 100% of the time.

Yes, the WRC guys use narrow tyres in snow, and often studded to get them extra grip in the icy ground underneath, but they're on almost 100% throttle all the time and have the road to slide in and push the studs to grip in.

I'd go with the approved sized from Audi if you feel the need to do winters.

I'd agree with above recommendation to use the size quoted on the tyre pressure label. You'll have no issues finding them with the correct load index and speed rating and they will be infinitely better than a summer tyre as soon as it's wet/cold/icy. The speedo will read correctly as well (within 1 or 2 mph)

I tested our S5 speedo on the factory 20s against the new 18s I bought and on the 20s the speedo agreed exactly with GPS. On the 18s the speedo suggests I'm going 1-2mph faster than GPS - error gets bigger the faster you drive