Originally Posted by BGTV8

My calcs show 255/30R18 are 610mm in diameter and 275/40R18 are 677mm so your differences are 66mm (about 2.65 inches) which is massive !!

Your TC/VDC will likely be FUBAR'd (and I'm not sure you can purchase a 255/30R18 without checking).

Did you mean 255/40R18 which is 661 mm dia?

Even so, you are 16mm out which is 2.5% difference and on the cusp of acceptable differences .... it might be OK, if might give you trouble.

Google "site: the370z.com tire size VDC" and review the results .... if you are going be this far, be aware you might have a problem.

Check the threads thrown up by Google and make your own decision but if you have 255/30R18, you will definitely have a problem..

If you want more "rake", then lower the front with your existing tyres - plenty of lowering springs available, but I'd also be wary of camber/caster/toe changes with a 1.5" change in rake - especially lowered at the front, because you will also affect the roll centre

Tread your own path dude.