can anyone tell me how to determine the biggest wheel diameter you are allowed to put on your car in victoria to stay legal? thanks

You can go any sized wheels as long as the overall diameter doesn't change over stock by more than 15mm, and 1 inch wider.
So a TJ comes with 16x7 standard, you can go 17/18/19/20 x 8 as long as the tyre profile is correct and has the correct load rating.

TJ Exec comes with 16x6 standard.

... let the debating begin.

You can go any sized wheels as long as the overall diameter doesn't change over stock by more than 15mm, and 1 inch wider.
So a TJ comes with 16x7 standard, you can go 17/18/19/20 x 8 as long as the tyre profile is correct and has the correct load rating.

The VRX/Sports/Ralliart and GTV came with 17x7, others came with 15x6 and Veradas
came with 16x6. You also need to check with your insurer to make sure that they will still
insure you with bigger wheels on. Insurance companies generally don't have a problem with
17", some may hesitate with 18" and most won't allow 19 or up.

can anyone tell me how to determine the biggest wheel diameter you are allowed to put on your car in victoria to stay legal? thanks

Dude go use the search function....there has been a million threads about this with people killing each other and arguing. If in doubt, call up VicRoads or go the the site.

TJ Exec comes with 16x6 standard.

... let the debating begin.
Yes but factory optioned for 16x7, and they're actually 15x6 for the Exec ;).

This has been covered a million times. Please use the search (http://www.aussiemagna.com/forums/search.php) feature.

p.s.
To save you some time, look at the following link.
http://www.aussiemagna.com/forums/showthread.php?t=44610