For anybody that could possibly care about my decision process for my winter tires, this thread's for you. For everybody else, im sorry for taking up internet space! :-)
I have a 2018 F80 ZCP, and was in pursuit of winter tires. My initial desire was to keep the 666M wheels - partly because I love the wheels, and partly because i like 20 inch vs 18 or 19 visually.
Of course, BMW recommends a square setup 255/35/19 on 19 inch 641 wheels, at least that was the package my local dealer was offering.
Quick background: this M3 is my extra car. i have two other cars, including an SUV that i can use when bad weather hits. So anytime my M3 sees snow it's because i want it to see snow for fun. therefore, maximum snow and ice traction is the lowest priority for me. i am after cold and dry performance. On the top of my priority is looks.
Here's what I had it narrowed down to:
1. Buy a winter set of 19's (Enkei Raijin 19) with Pirelli Sotto II's in 245/35/19 and 265/35/19.
2. Same wheels, but with Michelin Alpin PA4's in 255/35/19 and 285/35/19.
3. go with the BMW square setup using 255/35/19 all around.
OR
4. Keep 666M wheels, and wrap Michelin Alpin PA4's with 245/35/20 and 275/30/20.
5. Keep 666M wheels, and wrap Pirelli Sotto 3's with 265/30/20 and 295/30/20.
6. Buy a dedicated set of 20's and go with either of the above tire options.
I ended up going with option 5. And here's why.
First and foremost, visually it looks the best for me. I love the 666M wheels and i dont want to remove them for 19's that i dont love. and since snow performance is the least important factor, im not losing anything in the decision to keep the 20's. it also happens to be cheaper since i dont have to buy wheels.
Possible concerns: (1) mounting and dismounting tires twice a year and the potential for causing damage to wheel or tire. To manage this, I will be bringing my car to a local and reputable ferrari/exotic specialist that handles super low profile tires all day long. they quoted me a competitive price to do the tires, and i imagine that of all the places mostly likely to do a good job and be most careful with the best equipment, it would be a place like this. i realize it's still somewhat of a concern, but its low on my worry radar due to the specialist doing the work. and (2) the added cost twice a year to actually do the work. i'm very much not concerned with this, but i acknowledge that it exists. and also (3) not having the flexibility to swap out the wheels on my own time each season. That is certainly a cool perk of having dedicated wheels, but that alone didnt move the needle enough for me to care about that.
Another priority for me is potential traction control interference. i am told, and i know there's a lot of debate about this, that when the rear tires are smaller than factory, they are actually spinning faster... and this is why traction control kicks in because it think the rear is spinning faster than it wants and it attempts to limit the rear. Hence when you go from 285's in the back to a square setup with 255's, the 255's are spinning faster at any given speed vs. 285s. BMW certainly doesnt care, because if anything, it'll make the car a little safer. And frankly, that is probably helpful more than anything if the car is your DD and you commute, or rely on it every day of the year. For me, however, that didnt apply. not to mention i dont like the look visually of square 255's all around. rear looks too skinny for my eyes.
Factory total diameter for front and rear is about 26.3 front and 26.8 rear with some variation depending on manufacturer. with my chosen setup, im at 26.3 front and 27.0 in the rear. so with the rears actually being .2 inches larger overall, they are spinning a little slower, in theory there should be no impact on traction control (traction kicks in when it think there's slippage in the rear because they're moving faster than it should).
going with option # 2 above, the total diameter is actually almost an exact match to factory. which was compelling. coming in right about 26.3 and 26.8. So traction control wise and similarity to factory, this seemed like the best option but i still didnt want to go to 19's for 5 months of the year.
With option # 4 above, the front and rear overall diameter was almost identical to each other. and since the rears wear faster generally, it would only be a matter of time before the fronts were actually larger than the rears in overall diameter. which is not ideal. and could be potential cause of traction control interference.
Option # 6 was cool. But very expensive. The wheels that I want in 20" are just too expensive to justify. But if money was not a factor, I probably would buy a set of high end 20's and wrap these Pirellis on them. But I'm not trying to throw $4000-5000 (or more) on this decision
Anyway, that's my story. much of this information i cite above is from various sources such as local tire shops and the friendly tirerack staff. so my apologies if any of the information above isnt exactly correct. i didnt do any personal scientific research. but i trusted various sources and ended up making my own decision based on the information i had available to me.
Curious to know if any of you are as strange as i am when it comes to making decisions like these!! :-)
drive safe this winter! i'll post pics of my setup soon.
PS: i posted on a few previous threads that i was waiting on Michelin to release Alpin PA4's in 265/30/20 to match the existing 285/30/20... i was hopeful these would be released soon, but after continuous follow up it seemed like they would be available in the US until sometime next year at the earliest. So i gave up on that waiting game.