I would say that for the fronts (8.5J) you need a 245 wide tyre, so 245/40/18 would be pretty spot on (only -1.06% from stock, 653 mm rolling diameter as compared to 660 mm stock)
Backs (9.5J) you would need probably a 265 wide tyre, so 265/40/18 would again be closest (only -1.33% from stock, 669 mm rolling diameter as compared to 678 mm stock) or you could go for 275 wide (275/40/18) which would be spot on compared to stock. Depends how stretched you want the tyre over the width of the wheel. Adam at Z1 probably would give you better guidance.
Certainly the tyre sizes they come with are not correct (the backs are over 5% different, and the fronts 2.27% different from stock). Hope that helps?
Thats a great help thanks, I just wanted some to confirm my thoughts that the tyre sizes didn't look right. So do you think a 265 rear would look too stretched? Was definitely thinking that a 245/40/18 front was right, and like you have said would go for a 265/40/18 rear.
Thanks again for the help.