[bob.g]

i have a 2009 bmw 335xi with the 18” style 189 wheels.

i need winter tires and saw some 235/40/18 at a good price but when i check the offset calculator the rears seem to be above 1% (Was like 1.34 i think). Am i doing something wrong? i seem to recall someone saying if the tires are the same front to back, it wont be a problem with the xi system as long as they fit (and that size does fit based on posts ive seen, i just cant seem to find posts of ppl using them on an xi). Thanks!

[Priority 1]

What you need to worry about is the difference in rolling diameter between each of the tires mounted on the car at the same time to protect the transfer case, and the difference in rolling diameter between the new tire size and stock to keep the speedometer readings consistant.

People run square tires on staggered wheels with X-drive a lot and I am not aware of any issues with that. The only difference will be in appearance of the angle of the sidewalls which some people care about, and a slightly increased (~.200") section width (distance from sidewall to sidewall) in the rear versus the front. I haven't found anything that tells me that this affects the rolling diameter.

That difference you calculated tells you that the difference in rolling diameter from stock to winter will affect the speedometers reading. Since those wheels are quite a bit taller than stock they will travel further per rotation and your actual speed will increase relative to what the speedometer says with the stock tires. I found that the stock speedometer calibration reads higher than actual and so this may push your speedometer to reading closer to the actual speed. Make sure to keep an eye on it though and try to check it with a radar sign if you have those up north, and remember the delta between observed and actual speeds.

Off topic but in Canada I'd think you'd prefer narrower tires to help in deeper snow and 225/40R18 should be cheaper all around and keep the speedo consistant from stock to winter.

[Raptor04]

I am running 225/45/18 front and 245/40/18 rears non run flat winter tires on stock 189s. As long as your within 3% of stock diameter, you will be fine.

[bob.g]

really, its as much as 3%? i remember reading 1% tops.

[wdeerfield]

keep it to 1%. 235/40/18 on a 8 and 8.5 should have the same diameter i believe. i have 235/40/18 on my 18x8.5s for summer and 225/45/17 on my 17x8s for winter

[bob.g]

Originally Posted by wdeerfield

keep it to 1%. 235/40/18 on a 8 and 8.5 should have the same diameter i believe. i have 235/40/18 on my 18x8.5s for summer and 225/45/17 on my 17x8s for winter

yeah that's what i have, 18x8 front which is fine, its the 18x8,5 rear on this tire size that gives me problems.

when i check the calculator, if i set 8.5 wheel width inches, and 37 mm offset (i set that for both "current" and "new") then choose the original tire size of 255/35/18 and then put the NEW tire size of 235/40/18 and it gives a percent difference of 1.472%. it seems any "original" tire size i put for the rear always goes above 1% when trying to compare it to 235/40/18. Am i doing it wrong? i hope i am..will save me a lot of problems trying to find other tires.

[Priority 1]

Originally Posted by bob.g

yeah that's what i have, 18x8 front which is fine, its the 18x8,5 rear on this tire size that gives me problems.

when i check the calculator, if i set 8.5 wheel width inches, and 37 mm offset (i set that for both "current" and "new") then choose the original tire size of 255/35/18 and then put the NEW tire size of 235/40/18 and it gives a percent difference of 1.472%. it seems any "original" tire size i put for the rear always goes above 1% when trying to compare it to 235/40/18. Am i doing it wrong? i hope i am..will save me a lot of problems trying to find other tires.

Did no one read my response? If you are putting 235/40/18 on all four wheels there is a negligible difference in rolling diameter and you are fine in terms of your all wheel drive. Just keep in mind you are going to be driving faster than the speedo indicates, and could get a ticket because of that difference you calculated. If you want to keep the same speedo reading and get better deep snow traction, mount 225/40R18 on all four wheels.

[bob.g]

Originally Posted by Priority 1

Did no one read my response? If you are putting 235/40/18 on all four wheels there is a negligible difference in rolling diameter and you are fine in terms of your all wheel drive. Just keep in mind you are going to be driving faster than the speedo indicates, and could get a ticket because of that difference you calculated. If you want to keep the same speedo reading and get better deep snow traction, mount 225/40R18 on all four wheels.

haha sorry P1. i did read it, speedo isnt an issue for me, and id do 225 but everyone jumps on that size locally so i can score some used 235's dirt cheap.. but i wanted to double check to see the % diff since i keep hearing 1%+ is bad, and when i checked it, it was over 1%.

[wdeerfield]

obviously there will be a difference over the 255.. but all four wheels for the winter are the issue, front and rear. 235s front and rear will be fine. speedo will read 60 and youll be doing 59mph. 235/40 is .5inches taller than stock so thats why you see the % difference.

[Priority 1]

Ok so now that we have more information...make sure that the tread depth on those USED tires are identical. If they are substantially different, say 2 or more 32nds of an inch, then you could get into a problem with excessively wearing your transfer case.

If they have the same tread depth then you are good to go. I actually have the same issue right now in that i need to replace my LM-25 blizzaks that i got in 225/255 stock staggered sizes. They don't make those sizes anymore so I'm trying to warm up to the idea of the same tire on all wheels. I just like the look of the 255 winters in the back!

[Chrisred]

I just installed 235/40/18's all around for my winters on oem 189's. Feel great. I don't get that clunk anymore I used to get before running 235/35/19 & 265/30/19 when I was switching gears......

I went with 235's cause the 8.5 rears would of been tight with a 225

[bob.g]

ah ok. Thanks guys. so i guess ill go with these.

[07lilredwagon]

Originally Posted by Priority 1

Ok so now that we have more information...make sure that the tread depth on those USED tires are identical. If they are substantially different, say 2 or more 32nds of an inch, then you could get into a problem with excessively wearing your transfer case.

If they have the same tread depth then you are good to go. I actually have the same issue right now in that i need to replace my LM-25 blizzaks that i got in 225/255 stock staggered sizes. They don't make those sizes anymore so I'm trying to warm up to the idea of the same tire on all wheels. I just like the look of the 255 winters in the back!

Are these cars seriously THAT sensitive to tread depth variations!!!!!????? REALLY?? when we are talking about 32nds of an inch that is hard to see let alone calculate....this seems excessive to me from a common sense point of view. A car should not need this kind of fiddly attention....

The question for me is "why". is it because the drive shafts are rigidly mounted between the front drivers and the rear drivers through the transfer case?...Thats the way it looks from underneath the car. I would think BMW would have added some parameters to the design, but maybe they already did and their parameters are what they feel are acceptable at plus or minus 1 percent. That's tight!... Is this the same for other all wheel drive vehicles out there?

So if my rear tires are worn more than my fronts, for instance, are you saying that I will not be able to get close to the wear indicators on those rear tires if the fronts are in better shape? I would like to feel that I have gotten good use out of the tires and not have to worry about my transfer case blowing up or shredding its internal organs.

without re reading through the post...what is that magic number for the difference in tire diameter?.....1/16"?, 1/8"?, 1/4"?

So I guess I cant go with Cragar mags in the rear and skinnys up front?...My redneck BMW dreams shot to heck

seriously, I dont mind running the same size tire at all 4 corners...nice to know that we could, though....I guess thats for the 2 wheel drive crowd, eh?

sorry, Im a little punchy after last night!

JP

[Rich_NYC]

I just got these http://www.e90post.com/forums/showthread.php?t=785853 for my 328xi 19" 8.5 front 19" 9.5 rear et35 . what tires would be ideal for them ? ( have 07 328xi ) would 225/35/19 front and 245/35/19 back ( with 20mm spacers on the back ) give any problems ? thanks