Aspect Ratio: 40 or 45 for winter?

MetalMK · 09-04-2019, 10:49 PM

If I put 235/40-R18 on my M2, the front wheels will have a -1.6% difference, and the rear wheels will have a -3.4% difference.

But if I put 235/45-R18, the front wheels will have a +1.9% difference, and the rear wheels will have 0% difference.

However, BMW manual only provides data for 235/40-R18, maybe because BMW wants us to put 235/40-R18 for winters. If I go for 235/45-R18, I will not only save a few bucks, but also have less speedometer error. So which one is better? Thanks in advance.

bentom2 · 09-05-2019, 12:23 AM

Tire rack recommends a 245/40r18 for their wheel and tire packages. That’s what I purchased last fall.

235/45r18 might be too tall. If it’s the same diameter as the stock rear tires, most, including myself, have had rubbing in the fender liner with the wheel turned either direction. Since the winter tires you’re looking at are 20mm narrower, maybe they’ll clear? I can’t say.

Nikkize · 09-05-2019, 04:28 AM

Quote:

Originally Posted by bentom2

Tire rack recommends a 245/40r18 for their wheel and tire packages. That’s what I purchased last fall.

235/45r18 might be too tall. If it’s the same diameter as the stock rear tires, most, including myself, have had rubbing in the fender liner with the wheel turned either direction. Since the winter tires you’re looking at are 20mm narrower, maybe they’ll clear? I can’t say.

+1

mr_lab_rat · 09-07-2019, 11:55 PM

245/40 is happy medium

MetalMK · 09-08-2019, 05:36 PM

Quote:

Originally Posted by mr_lab_rat

245/40 is happy medium

But it's not what is written on the owners manual. And from what I've heard, narrower winter tires are better to dig into the snow and get traction?

CTSean · 09-09-2019, 07:53 AM

In my personal opinion, I can't imagine anyone but maybe a racing driver would notice the difference between 235's and 245's. I'd rather match closer to the stock diameter than worry about the line in the manual about using 235/40's

Jut for reference, I have 245/45'/18's for winters because that's the size that came with the lightly used set I bought. There work fine and they only rub a tiny bit on full lock. when I need a new set, i'm thinking the 235/45/18's might be a good choice since it;s a touch more ground clearance and it wil make a better spare when the summer wheels and tires are on (since it matches the rear diameter pretty much perfectly).

09-04-2019, 10:49 PM	#1
MetalMK First Lieutenant 229 Rep 328 Posts Drives: BMW Join Date: Dec 2017 Location: Canada iTrader: (0)	Aspect Ratio: 40 or 45 for winter? If I put 235/40-R18 on my M2, the front wheels will have a -1.6% difference, and the rear wheels will have a -3.4% difference. But if I put 235/45-R18, the front wheels will have a +1.9% difference, and the rear wheels will have 0% difference. However, BMW manual only provides data for 235/40-R18, maybe because BMW wants us to put 235/40-R18 for winters. If I go for 235/45-R18, I will not only save a few bucks, but also have less speedometer error. So which one is better? Thanks in advance.
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09-05-2019, 12:23 AM	#2
bentom2 Lieutenant 336 Rep 427 Posts Drives: 2018 M2 Join Date: Jun 2018 Location: Redmond, WA iTrader: (0)	Tire rack recommends a 245/40r18 for their wheel and tire packages. That’s what I purchased last fall. 235/45r18 might be too tall. If it’s the same diameter as the stock rear tires, most, including myself, have had rubbing in the fender liner with the wheel turned either direction. Since the winter tires you’re looking at are 20mm narrower, maybe they’ll clear? I can’t say.
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09-07-2019, 11:55 PM	#4
mr_lab_rat Major 564 Rep 1,092 Posts Drives: 2017 M2, 2018 X1 Join Date: Jul 2017 Location: Vancouver iTrader: (0)	245/40 is happy medium
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09-09-2019, 07:53 AM	#6
CTSean First Lieutenant 323 Rep 392 Posts Drives: LCI BSM MT M2 Join Date: Dec 2018 Location: CT iTrader: (1)	In my personal opinion, I can't imagine anyone but maybe a racing driver would notice the difference between 235's and 245's. I'd rather match closer to the stock diameter than worry about the line in the manual about using 235/40's Jut for reference, I have 245/45'/18's for winters because that's the size that came with the lightly used set I bought. There work fine and they only rub a tiny bit on full lock. when I need a new set, i'm thinking the 235/45/18's might be a good choice since it;s a touch more ground clearance and it wil make a better spare when the summer wheels and tires are on (since it matches the rear diameter pretty much perfectly).
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