Jz39

iTrader: (2)

Hi all.

I've been going through the motions of getting new tires and wrestling between the stock and 245/275 combination on 2011 wheels.

After doing a fair amount of research on this, I ended up with an intermediate selection that seems to be a good compromise.

I decided to go with 235/40 PSS up front, exactly 0.3 inches greater in diamter than stock.

The rear tires is where it was more interesting. There are a bunch of different OEM options for PSS in the 265/35 size, but the one that was most intriguing was one made for Porsche that actually had the same tread width (10.1") as the PSS 275 size. I liked this because I didnt want the tires to bulge, yet it still has the apparent advantages of the 275 in terms of tread width.

The other thing I liked is that both tires are 0.3" greater in diameter than stock. Given how low profile the tires already are, it seemed that 0.3" increase was a nominal amount in the right direction.

Anyone else thought of this combination or reasons why it would not be a good idea?

I think if I had the 2012+ wheels I would have gone 245/275, but Im hoping this ends up being a good match.

SubOrbital

You'll also notice N0 spec (aka Porsche) spec tires also include a free 1/32" reduction in tread depth.

jspecvtec

iTrader: (4)

Seems fine to me. I might do they same but I have 2012+ wheels so I'll decide when my tires get bald

lobuxracer

iTrader: (2)

Hopefully mechanical performance is not important.

Jz39

iTrader: (2)

LOL...isn't that nice of them! Funny thing is that all of the other similar PSS 265s with 10/32" were priced at $329-355 and the NOs were only 301$, so you give up a 32nd and get 30$ or more off per tire. So you give up 3% in tread but get 10+% back in price.

Jz39

iTrader: (2)

Hey lowbuxracer. As a newb here I've learned a ton about the IS-F from your posts!

But I'm not following the logic of mechanical reliability here.

The 235/265 set up seems to have some minimal advantages over the 245/275 or greater set up in that:

1) the 235/265 is well within the recommended range for the 8 and 9" rims
2) the difference vs stock diameter is a uniform +0.3" front and back, while the 245/275 is -0.3 up front and -0.5 in back
3) the rears weigh a pound less than the 275s but have the same tread width as the 275s if going with the NO Porsche specs
4) revs/mile are closer to stock and front to rear than the 245/275 setup
5) bumping up to 285 is outside of the "theoretical" range for a 9" rim

What are you thinking would cause mechanical unreliability?

lobuxracer

iTrader: (2)

Not reliability. Performance. You are changing the suspension geometry pretty dramatically with tires in these diameters. Scrub radius, roll center, center of gravity, instant centers, roll couple...all of it changes when you deviate from the OEM diameters. When you deviate this much, it can noticeably reduce mechanical grip.

jspecvtec

iTrader: (4)

Yeah wider isn't always better especially since 235/265 is the recommended sizes on your rim width. You also won't necessarily have more traction going wider unless you actually have a higher measured contact patch. However, You'll have better steering response on that setup than the 245/275. Let's see what the experts have to say....

lobuxracer

iTrader: (2)

I disagree the steering response will be better. If the diameter was correct, I would agree, but there will be a discrepancy between tire center and center of rotation on the kingpin axis because these tires are not the same diameter. This is what scrub radius means, and it really changes the feel of the steering and can easily negatively impact mechanical grip while turning.

Jz39

iTrader: (2)

Steering response was another reason I went with just the one upsize vs two upsizes in width. I've got 245/275 PSS on my G Coupe and when I put my winter 225/245 tires back on its amazing how much more responsive and light the steering is.

I think in the long run that's why I was looking to split the difference, a bit more grip with a bit more steering weight.

Agreed fully on the changing geometry, but it would seem that any change from stock is going to do that, no?

I get these on next Saturday 7am with alignment and I'll takes some pics and let you know my initial thoughts.

Thanks for the comments!

Jz39

iTrader: (2)

Any thoughts on target alignment specs?

lobuxracer

iTrader: (2)

You can only adjust toe unless you have bought and installed aftermarket devices to allow adjustability.

Jz39

iTrader: (2)

I'm interested to see what my before alignment specs are for the worn stock size PSS on the car. There is some vibration around 85mphand it doesn't feel like camber wear, I'm thinking more along the lines of toe.

Lowbuxracer--what do you recommend to hit for toe?

Jz39

iTrader: (2)

This being another reason why I was hesitant to go wider, as wide tires tend to exacerbate alignment issues. I almost went stock size to check everything out, alignment etc, but the car really does need a bit more meat up front and in back.

jspecvtec

iTrader: (4)

Yeah because tires too wide on the rim width will cause it to flex over the rim too much and make the sidewall rubbery and less stiff. So steering response is slower and heavier