on a new tire,,,that's about 0.7"
probably close,,,,some get away with that much.
If the tires are used/measure the circumference & divide by pi.
put them on....i want to see.

New Z owner. 2016 Nismo. Think I'll keep the stock rims but will powder cost them flat black. I want to move the 285's up front, tire rack says they will fit the 9.5 inch rims. I want to throw Pilot Cup 2 315's on the back. Guessing I'll need mild spacers on both?

15/20 H&R DRS yep

Need spacers that big? Is it going to stick out very far with that setup?

You can do 10/15 if you like are you planning on lowering the car?

Maybe a bit with Coilovers down the road or I could get the HR sport springs if that would work.

I want meaty tire but want the best handling I can get too

The "Will it fit?" thread
 

I ran swift springs (they'd get my vote again) and 20/25 spacers with no issues.

That was on sport wheels. The new nismo wheels are .5 wider so that's why I suggested 15/20 they'll sit pretty much at the edge of the fender.

If you do lower it the negative camber that comes along with the drop will tuck them nicely on 245/285.

If you're hell bent on doing the 285/315 combo I'd swap the tires frist then measure then get the appropriate size. That fat you might end up with 10/15.

If you are going to run 285's up front you don't really need spacer, at least nothing more than 2 to 5 mm.

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I would rethink your choice of tires. With Pilot Cup's on the rear. You will get lots of understeer when the tires come up to temp. And they are short lived on the street.

He may be going for the "Look" Russ,
as i would never run 2 different brand/tread compound tires on my ~Z~
Unless i was to bolt on some Drag-radials

No, not going for looks with a certain brand of tire. I want fat tires because I like they way they look but that secondary to performance I suppose. I just know that the pilot cups are hard to beat. I'd probably want them in the front too. Just right off the bat I figured why not move the current tires I have up there. But understeer is a good reason not to.

If you are not going to track the car there is no need to get such an expensive set of tires. There are plenty of really good street tires that will accomplish what your goal, which I assume is form over function!

Function is the main goal. They might be a more expensive option than what's needed for street but they are the best and a set I've always wanted. I do plan on having two sets of wheels/tires and I do plan on tracking. Might be overkill and needless for the streets but we want what we want right? Doesn't have to make sense. Having said that I'm totally open to the PSS instead from and rear for the street. 275's up front and 305's out back. I'll still have a set of the cups, maybe only for track days.

There's a lot going on here mixed tread patterns sizes and spacers brands... OP I think you are better off just swapping the rears to the front getting a matching set of 325/30s for the rear, drive the car for a while then see how it goes. Then add spacers springs yada yada yada.

Unfortunately they don't make a different size in the Bridgestone S001 that my Z came with. But I like that idea