So I'm looking at 18x8 AR IROCs, with the 4.5" backspacing and 0 mm offset to replace my stock 15x7 RS wheels.

Below is a diagram of my RS wheel, showing 4.5" backspacing via a google search, and a 12 mm offset via a chart in the sticky at the top of this forum about measuring backspacing/offset http://www.rsracing.com/tech-wheel.html#backspace , which sounds right since 12 mm is about a half inch.



As you can see, on the stock RS wheel, there are 3.5" from the mounting flange to the outer edge of the wheel, and 4.5" from the flange to the inner edge of the wheel.

Here is the diagram for the American Racing IROCs:



On this diagram you can see that the backspacing is again 4.5" (so the inner edge of the wheel will be at the same depth into the wheel well as the stock wheel), and the distance from the mounting flange to the outer edge of the wheel is 4.5", one inch more than stock.

1) Thus, if I'm thinking about this correctly, this means that the inner edge of the IROCs will be in the same position as the stock wheels, but the outer edge of the wheels will stick out one inch more than stock, closer to the fender lips. Is this correct?

2) I'm running Eibach Sportlines to lower the car. Given that the car is lowered, what are the widest tires I can safely fit with NO RUBBING whatsoever? I want to remain at a 26" height. Ideally I would like to run 255/40R18, as this is a 26" tire. I'm not sure if this is going to fit without issue, or if I can even maybe go wider with the Sportlines? I'd love 275s all around, but I don't know that I can that so I was thinking 255s.

Sorry for the long post, been researching and using available resources to try and figure these things out to make sure my understanding is right. I know the IROCs will fit and fit well, but I want to actually understand where that extra inch of width is being put, and try and figure out the fattest tire I can stick in there without rubbing since the car is lowered.

Hopefully there are some people running sportlines and 18x8 IROCs.

EDIT: Also if someone could confirm the measurements of the RS wheel, I'd appreciate it.

 

245/40/18 for the front, you can go a little taller in the rear with no problems. the 18x8 with zero offset will fit just fine.

you cant put a 275 on a 18x8, the wheel is no wide enough. You pretty much stock with 245, or maybe a hair wider.

 

Think I could do a 255/40 or is 245 gonna be the limit?

Also do you think the Sportlines will affect things or change anything?

 

You may have to roll your fenders, but most good wheel shops have the right tool.
255/40 should be fine out back. Try one of the rears on the front before deciding what to put on the front.

 

Definitely not willing to roll fenders, and want to run the same size tire on all corners.

 

Nobody with IROCs and Sportlines??

 

my brother has 18" IROCS and Sportlines....



I used to have 18/20 combo IROCS on mine with sportlines...never had any problems...

 

what size tires did you and your brother run and any rubbing issues at all for your brother?

 

His specs are:

18X8 all around, 0MM offset, 4.50 backspacing, tires are P245/40R18 all around. No rubbing issues at all.

I can't remember what I was running....it's been to many years ago to remember.

 

i had sportlines and 18" irocs on my 92 Z convertible. 245 tires front and back. they were the american racing wheels. not a single issue. buy them and put them on, end of story