[martopoulos]

So, I had a stock, square setup with run-flats and I've replaced the wheels with 18x8 ET 35s all-around. I'd like to get wider tires to replace the stock 225/40R18s, but I've read conflicting information about issues with DSC. Will I be okay as long as the tire diameter is the same in the front and back since my car was calibrated for a square setup? Or am I misunderstanding how this works? I was thinking 235/40R18 all-around (or 245s, if I can get away with them in the front - everything is stock except the wheels).

[Cyberdemon]

I run 235/40R18's all around no issues at all.

The DSC should not have an issue with minor variations. Even the stock 245/35R18's are slightly smaller diameter than the fronts.

[martopoulos]

Originally Posted by Cyberdemon

I run 235/40R18's all around no issues at all.

The DSC should not have an issue with minor variations. Even the stock 245/35R18's are slightly smaller diameter than the fronts.

Would 235/40s in the front and 245/40 in the back be too much of a variation if my stock setup was tuned for square/225s all-around?

[Beartato]

I run 235/40R18 on 18x8.5 wheels and haven't had any issues. What issues are we talking about?

[martopoulos]

Originally Posted by Beartato

I run 235/40R18 on 18x8.5 wheels and haven't had any issues. What issues are we talking about?

Apparently, if you deviate too far from the stock tire diameter, you can have DSC fire prematurely, is my understanding. But it might be that too great a deviation between front/rear diameters vs. stock is the real issue, rather than simply increasing the size all-around like on a square setup.

[Zooks527]

Originally Posted by martopoulos

Apparently, if you deviate too far from the stock tire diameter, you can have DSC fire prematurely, is my understanding. But it might be that too great a deviation between front/rear diameters vs. stock is the real issue, rather than simply increasing the size all-around like on a square setup.

DSC only cares about relative rotational speeds of the wheels (which ties to tire diameter) across the axles. It can't tell if your tires aren't stock, only if they differ from front to back.

This highlighted section above is what counts. The car's ECU runs a setup sequence when you start driving to try to determine how much variation there is in the setup. The system can easily handle one percent variation in diameter. There is some evidence that 2% or so is ok, but there's no hard documentation that I know of for that.

[DanG]

One thing to consider is that 225/40/18 is the recommended size for an 18X8 wheel.

I am running 225/40/18s on my BBS CH-R 18X8 square wheels.

[martopoulos]

Thanks, guys

[XutvJet]

Originally Posted by DanG

One thing to consider is that 225/40/18 is the recommended size for an 18X8 wheel.

I am running 225/40/18s on my BBS CH-R 18X8 square wheels.

245/35 is the stock size tire for the rears on cars equipped with staggered rims and MSS tires. A 245/35 is more than safe on a 18X8 rim. Same goes for a 235/40 and even a 245/40 if it would fit the body. 225/40 all around is simply too narrow for a 3,450-3,600 sports coupe, IMO.

[Joe240]

Originally Posted by Zooks527

The system can easily handle one percent variation in diameter. There is some evidence that 2% or so is ok, but there's no hard documentation that I know of for that.

The factory staggered setup is 225/40 245/35. That's a difference of 1.35% so I think it's safe to say that at the most, a 1.4% variation in rolling diameter is safe for the DSC. Could always go 225/40 255/35 and make that .24% rolling diameter difference.

[BPAC306]

Originally Posted by Beartato

I run 235/40R18 on 18x8.5 wheels and haven't had any issues. What issues are we talking about?

do you have a picture of how your sidewall looks on the rim? I am looking at 18x8.5 with a 4 corner setup but don't know if I should do 235 or 245. I want some rubber to overlap the rim.

[Beartato]

Originally Posted by BPAC306

do you have a picture of how your sidewall looks on the rim? I am looking at 18x8.5 with a 4 corner setup but don't know if I should do 235 or 245. I want some rubber to overlap the rim.

It overlaps a little. I'm pretty sure a 245 wouldn't fit in front without a spacer, as a 235 is really close.

[DanG]

Originally Posted by XutvJet

245/35 is the stock size tire for the rears on cars equipped with staggered rims and MSS tires. A 245/35 is more than safe on a 18X8 rim. Same goes for a 235/40 and even a 245/40 if it would fit the body. 225/40 all around is simply too narrow for a 3,450-3,600 sports coupe, IMO.

Michelin totally disagrees with you. 225/40/18 is their ideal size on an 18X8 rim.

[XutvJet]

Originally Posted by DanG

Michelin totally disagrees with you. 225/40/18 is their ideal size on an 18X8 rim.

They should call up BMW and tell them they're doing wrong as my OEM 18x8 rims came with 245/35r18

[JoeyF228]

Originally Posted by DanG

Michelin totally disagrees with you. 225/40/18 is their ideal size on an 18X8 rim.

Ideal doesn't mean required, i've run 8" wheels on 235/40's on a few cars with no issues. Not all tires run the same width either, a 225/40 Pilot super sport or other extreme performance tire sometimes have a wider tread width than a 235/40 all season.