In Victoria, the outside diameter of the tyre size cannot be increased by more than 15mm. Check out:
http://www.vicroads.vic.gov.au/NR/rd...340/0/VSI8.pdf
A well known tyre size calculator is available at:
http://www.miata.net/garage/tirecalc.html
Just type in the old and new tyre sizes and you can immediately determine the change in outer diameter and the effect that it will have on your speedo.