Assuming you are driving an N/A Crossfire and ignoring just how absolutely retarted 20x8.5 on all four corners is... You'll want to preserve the stock overall diameter as much as possible (due to wheel speed sensors, speedometer accuracy, and brake design)

And since I don't expect you to know how to work a tire caluclator, let me do if for you and post the sizes...

Rear = 255/30R20
Front = 225/30R20

That's preserving the factory overall diameter (within 2%), widths, and fitment assuming you did your homework on the rim offsets.

Good luck.