can you use a 275/40 tire on a stock 18x8.5" wheel?
 

2008 350z roadster

Can it be mounted? Yes.

Safely? No, it's pushing beyond the safe limit. Have to assume you want a size like that for "performance reasons". Performance and safety is compromised meaning it's not worth doing.

I'm running a 275/40/18 on the factory 18x8 wheel for that past few years. Cornering is fine. The front 225's will chirp on corners and the rears never chirp. Honestly probably not worth the extra weight as I'm only around 300whp. I'd probably go 265's if I had to do it again just for weight reasons only.

I would not run a 275/40 tire on a stock 18x8.5" wheel. The sidewall (and possibly the car's handling) is adversely affected by this deviant wheel/tire-size combination. You should mount at least a 9.5" wide wheel if you are running a 275 mm wide tire.