I'm running stock 18 x 8 1/2 with MT 265/40/18. They are almost stock size. Checked with GPS, no tune change and my speeo was one mph low vs GPS. You would think high but not according to my GPS. And I had the mph in my tune right on before the tire install using Nitto Invo 285/35/19.
2013 Cadillac CTS-V - 11.18 @ 125.22 mph
2012 Z06 - dyno 537/513 w/bolt-ons (sold)
08 A6 Corvette - 10.88 @ 125.25 mph, 1.56 60'.(sold)