Winter tires necessary?
 

You should not use summer tires in the snow. Get winters or high performance all seasons like Continental Extreme Contact DWS 06 or Michelin Pilot Sport A/S 3.

Summer tires also do not work under 40F. Like others say, get A/S. I went 265/40/r21s in p-zeros.

I think you'd be interested to see this thread:
https://www.audiworld.com/forums/q5-...setup-2936255/

I LOVE the M/S (mud/snow - not summer) tires that came on my Q5. I'd swear I can drive on glare ice... but snow is easy.

It seems to me that if the OP already has summers his second set doesn't need to be A/S--he should get winters like Blizzaks or Nokians

Not only do summer tyres not work in snow, as stated by jrm83, they do not work well in low temperatures either.

There are two factors at work here:

1. The tyre's compound can only be formulated to be flexible for optimum grip and wear in a relatively narrow temperature range. Summer tyres designed to cope with the maximum speed of the vehicle have to handle getting on for 200 degrees Fahrenheit without failing hence the compound used hardens substantially at winter temperatures. The impact of the compound hardening at low temperatures adversely affects grip in all conditions, even a completely dry surface. Winter tyres have specialised compound formulations that not only enhance mechanical but molecular grip with additives such as silica and are flexible in much lower temperatures. Winter tyres with compounds that remain flexible well under freezing will carry speed ratings of S or T as their compound and construction cannot handle the temperatures associated with higher speeds.
2. Tyre tread patterns also play a significant role in grip levels on snow and ice. Winter tyres have many extremely thin cuts in their tread blocks called sipes which greatly enhance their ability to grip in these conditions. Summer tyres do not have sipes as at higher temperatures and speeds, sipes are counterproductive as they reduce "feel" and increase wear.

All-season tyres, particularly those carrying W or Y speed ratings are simply a compromise. The compounds are formulated to remain flexible over a huge temperature range of in the region of 250 degrees Fahrenheit. They will exhibit less grip, sometimes significantly, than either a summer or winter tyre across this entire temperature range. Their tread patterns reflect the need to clear water, not overheat at high speeds, and offer at least some mechanical grip in winter conditions. That said, they generally offer more than sufficient grip within a temperate temperature range and enhanced grip compared to summer tyres in occasional light snow.

Absolutely. Why compromise with all seasons if they won't be run in summer. And I read the difference between a/s and winter tires in snow is as significant as the difference between summer and a/s in snow....meaning you'd be sacrificing a lot of perfomance, not just a little.

If he's after winters, then just grab a set of 19 or 20s. I went A/S because I didn't feel like going back and fourth to swap tires and storing them. I thought it was A/S or a dedicated set of winters. But if the OP wants dedicated winters, go that route.