So I am looking to buy a set of Tarmac II. Really wanted 17, but found 18 instead.

So this is what I have gathered so far...

Kumho ECSTA MX vs (225/40/18)
Bridgestone Potenza S-03 vs (225/45/17)
Bridgestone RE92 (205/55/16)

section width = 9.1 vs 9 vs 8.4
dia = 25.1 vs 25 vs 24.9

I know someone with Prodrive P1 17x7 (+52mm offset) with the S-03 above and rub with stock 00RS suspension, no fender roll, and no passenger.

The question is, if I pick up this set of Tarmac II 18x7.5 (+48mm) with the ECSTA above with AGX GR2 w/P1 springs on a 00RS, and no fender roll, will it rub?

 

okay, after some math, this is what I came up with.

Due to smaller offset, I gain an extra 4mm buffer from the fender.
Due to the bigger width on the 18" tires, I gain 2.54mm total, which is 1.27mm each side.
Due to bigger diameter, I gain 2.54mm total, which is 1.27mm up top.

So at the end, I would gain 2.73mm in width buffer but loose 1.27mm in height buffer.

Damn, I don't think that 2.73mm buffer would give me any room since my GR2 w/P1 actually dropped my car a little.

 

So looks like if I were to go with 17 with 48mm offset, I would gain 2.62mm outside vs RE92 on 16x7 +53mm.

with Prodrive P1 that would have been 6.62mm on the outside

Plus, with 17, I could go with 215 instead of 225...

can someone confirm my math and tell me if this really won't fit. Or tell me if you know what would fit with my setup. Thanks!

 

I would be inclined to think you'll be okay with that setup. First off, since your GR2s are the same length as stock when fully compressed, your springs will NOT affect whether or not your new tires rub. Suspension changes only come into play when you are using adjustable perch coilovers, or shortened replacement struts. But without them, your spring doesn't matter because you won't rub until you're on the bumpstops (or close) anyway. Anyways- 5mm give or take is a really small amount. You should have far more than that in clearance to the strut, and at least that in clearance to the fender. So depending on your camber, you might want to pull the rubber strips off of your rear fender, but I wouldn't expect to require rolling at all.

As for tires, a 215/40R18 would be ideal since 225/35R18s are ridiculously priced. It's a touch smaller than stock diameter and still wider. If it was me, I'd grab the BFG KDW2 in that size. It's cheap for an 18" tire, sticky, a great rain tire, can be tracked and autocrossed, and that model tends to run a bit wider than its size, so it would be as wide as many all-season 225s, but not as wide as the Kumho MX.

HTH dude.

 

I didn't know that... Thanks man! The reason I mention that tire model and size because it's what comes with the tires. I'll see if I can get a fitment test before I buy. thanks again!

Leo

 

Sorry for the short thread jack ~ but I decided to go with 215x40x18 ~ but I can also get 215x35x18's which would be a true +2 fiment. Is everyone using 40's because the ride just sucks on 35's? I have an NA car and would like to keep unsprung weight as low as possible (i.e. the 215x35x18's). Would this choice prove a mistake in the long run. Running 40's would also give me a slightly higer gear ratio... something I do no need on my NA car.

 

I think most people end up with 225/40 because that's the most common 18" size in an appropriate width. Plus, the slightly taller sidewall does help a little with ride quality I suppose.

 

BAN SUVS, do you personally recommend 18 at all? I had my heart set for 17, but just hard to find an used set of 17 Tarmac II Gold.

 

For an Impreza I would recommend 17s personally, but I don't think 18s are just plain dumb. The ride harshness is much worse if you go straight from stock RE92s to 18s, but for someone who already had 17s with good tires (read: stiff sidewalls) the difference isn't huge. And since the added rotating mass doesn't really matter to the vast majority of us, since we don't have anything but pride at stake when we are running against the clock, that's not a huge issue either. Basically, if you want 18s, get 18s.