Tires will be quite a bit smaller in diameter vs. your OEM size, fronts will be almost an inch shorter and the rears 1.6" shorter, not a great combo TBH, I think I'd change the rears to a 275/35 and keep the fronts if they're in good shape.
As far as how to tell if they're fakes, that wheel has been copied a lot, here's a real one w/ all the raised lettering in the barrel, if it's missing any of that lettering beware, and $800 for real SSRs I'd for sure be on guard. With that large positive offset on an 8.5 there may be trouble clearing the BBK, but you will have plenty of room to go out w/ spacers if needed.

 

How bad is it to have smaller diameter? Pros and cons?
What are correct tire height sizes for these wheels? I'd prefer to get wide tires, like 245/275 or even more.

 

Not too bad to have the smaller diameter, will ride harsher w/ less sidewall. Having the rear tire another 1/2" shorter than the front may caused issues but not sure since you're RWD. It would be better to have that 275 size w/ that 245 front though. On a 9.5 width wheel I wouldn't run more than a 275 for the best mount, you could run up to 285 though.

 

What is correct tire height? I currently have 225/50/18. Will 245/40/18 in front give me correct total diameter?

 

Use this calculator when trying new sets
https://www.myg37.com/forums/tire_rim_calculator.php
There's not an exact "correct" diameter, but your speedometer should be closely calibrated to your OEM tire, which in your case would be that 225/50
A diameter within 1% of that is fine, for RWD you can go up to 3% but w/ AWD you should be less than 1% and closer to zero ideally.
A 245/45/18 would be about 0.6% off from your OEM, the 245/40 would be off by more than 4% so that wouldn't be a good size.

 

Anybody running a F: 19x9.5 +20 on a 245/40/19, and R: 19x11 +28 on a 275/35/19... or something of the sorts? I know they'll fit with some camber and fender work if needed, but I'm more worried about inner clearance to the strut/coil body than anything else.... I don't want to run spacers! lol Thanks!

 

No worries on inner clearance w/ those offsets, if the offsets were >+40 range then it would get close. The rear w/ 275s will be tight, camber will probably have to be about -3 to tuck back there.
And I assume you have a coupe, for a sedan you'd need about 10mm to 15mm more room at the fenders.

 

Your rear tire size is incorrect. You can't go from a 40 series up front to a 30 in the rear...what you meant to say was 275/35.

 

I knew I was forgetting something lol yessir, it's a coupe. And thanks for the info!

 

Good catch! Yep, what he said^^^^

 

Yep, I meant 35 series lol thanks, guys!

 

Niche Misano
20 x 9 +25 245/35R20
20 x 10.5 +30 275/30R20

On a 2008 G37S Coupe. I would like to run these with no fender rolling or fender line chopping.
Cars on Stance coils but not a very aggressive drop as its a daily driver and I want to keep it drivable.

Will they fit an any idea on tires to fit these?

 

No idea, wish someone ran those sizes before now.

 

Rears will be tight so the bumper bracket may need a trim, but other than that should work ok. May have to run camber around -2 as well.

 

Just want to confirm with everybody that this will fit on coupe:
SSR type C
F: 18x8.5 +41 255/45/18
R: 18x9.5 +41 275/40/18
My mechanic says he's not sure if this will fit, so he refuses to install. And it's kinda scary to buy everything when there's no guarantee.