Tire size help

[08pearlZ06]

Ok guys. So I put 325/45/17 on the rear of my Z0. Put the size in the calculator. And from what I can tell. Speedo is off by like 2 mph. And cruise gets another mile an hour off per 10 mph you go. Ie 70 mph according to speedo. Set cruise. And says 63. Already have my seat in the June efi 101 class. Just need some help till then. Thx

[joecar]

Post your tune file...

and post the measured tire diameter, and the measured tire circumference (measured by rolling your car and measuring distance tire moves in 1 rotation).