I'm running these on my m6 stock 20's. 255 front and 285 rear but you can get 275 rears too.

-Pirelli Winter Sottozero Serie II(performance winters)
Front: 255/40/20
Rear: 275/40/20
Rear : 285/35/20

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2013 M6 Coupe SG, Akropovic Evo Carbon, VS Full Aero Kit & VS-3XX's 20's, Gruppe M Intake, Bel Stir Plus with Dual HP Laser Interceptors, JL Custom 2X12W6V2's Subs, 35% tints all around.