Just an FYI if anyone was wondering about this. I just installed 225/40R19 fronts (Bridgestone Blizzak LM-32), and 285/35R19 rears (Michelin Pilot Alpin PA4). The brand mix 'n match may not be right for everyone, but I'm confident that it will be fine. You might be able to find these sizes in the same brand of tire.

The overall diameter of the stock Michelin Pilot Supersports was 25.5" front and 26.2" rear, which is a ratio of 1.027:1 rear to front.

The overall diameter of the winter front tires is 26.1" and the rears are 26.9". This is a ratio of 1.030:1 rear to front which is close enough to stock IMHO. I calculated the circumference ratio as well, but I don't have those numbers handy at the moment.

I know it's not much difference, but it's a little taller and narrower than stock, which should help a little in the snow. If you were to go with the Porsche recommended 285 or 295/30R19 rears, you'd end up slightly lower than stock.

 

Here's a pic with the new tires.

 

Be careful until you get used to the handling. I've used Blizzaks and Alpins in the past (never mixed) and they are very different tires. Each has strengths and weaknesses.

 

Thanks for the advice, I'll take it easy at first!