18's tire size question 2kz28

I have a 2000 Z28, I have a BMR Adjustable Panhard Bar and Tubular Lower Control Arms, And I currently Have 275/35 R18 tires on 18x9.5 inch z06 wheels with 56mm or 54mm backspacing, and I was wondering If i could fit a little bigger sidewall tire without it hitting anything, it isnt lowered, its just i live in memphis and the streets are crapped, maybe a 40 series? anyone have anything like this?

people have been fine with the 275/40/18 tire.

http://www.miata.net/garage/tirecalc.html

Input the stock size that came on your z (245/50/16) on the left and on the right input the new tire size you plan on running. Going to a bigger or smaller tire can sometimes throw the speedo off.