New wheels require 5mm spacers

[demick]

It may depend on the wheels too. Some wheel designs have thicker bosses which the bolt must pass through - effectively shortening the bolt protrusion and threads which will be able to engage the threaded hub.

So you can't really just assume that you will have 5mm less thread engagement from your stock wheels. You probably need to wait until your wheels arrive and see how much thread protrusion you have with those wheels with 5mm spacers.

I believe that 1.5 times the bolt diameter is a safe number for proper thread engagement, but I'm not entirely sure.

[boxster6354]

Here is a link that will explain it:
http://www.performancealloys.com/alloy_wheel_spacers.asp