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Best tire size? 18x8.5” wheels.

TacoCarl

Member
Mar 24, 2021
5
1
Boston, MA USA
I’m wondering what alternative (from oem) size tire is the best fit for essentially the oem aero wheels. Performance and aesthetics are my main concern, and I doubt a slightly wider or narrower tire is going to have a huge effect on range.
I just bought some nice Martian wheels for my ‘21 M3, size 18x8.5”. I’m putting on Michelin pilot sport 4S tires but the size I want (235/45r18) is out of stock EVERYWHERE; so I’m looking for an alternative size. As of now I’m leaning toward a 245/45r18, but I’d love to know what people have run on their cars, what the differences are like compared to the 235/45, and just people’s opinions on the matter.
thanks!
 

MODEL+

Vendor
Oct 21, 2020
749
587
Portland, OR
I’m wondering what alternative (from oem) size tire is the best fit for essentially the oem aero wheels. Performance and aesthetics are my main concern, and I doubt a slightly wider or narrower tire is going to have a huge effect on range.
I just bought some nice Martian wheels for my ‘21 M3, size 18x8.5”. I’m putting on Michelin pilot sport 4S tires but the size I want (235/45r18) is out of stock EVERYWHERE; so I’m looking for an alternative size. As of now I’m leaning toward a 245/45r18, but I’d love to know what people have run on their cars, what the differences are like compared to the 235/45, and just people’s opinions on the matter.
thanks!
If you don't mind the range lost, you can use 265/40/18.
This will keep the same exact overall diameter as 235/45/18.
Your tire is going to sit blocky on a 8.5 though.

DSC_9318.jpg
 
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TacoCarl

Member
Mar 24, 2021
5
1
Boston, MA USA
If you don't mind the range lost, you can use 265/40/18.
This will keep the same exact overall diameter as 235/45/18.
Your tire is going to sit blocky on a 8.5 though.

View attachment 647633
If you don't mind the range lost, you can use 265/40/18.
This will keep the same exact overall diameter as 235/45/18.
Your tire is going to sit blocky on a 8.5 though.

View attachment 647633
I don’t think I’d want to go that big in width on an 8.5” wheel. I’m not super concerned with a minute discrepancy in speedo reading, but I think the range hit from a 265 might be a concern.
 
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MODEL+

Vendor
Oct 21, 2020
749
587
Portland, OR
I don’t think I’d want to go that big in width on an 8.5” wheel. I’m not super concerned with a minute discrepancy in speedo reading, but I think the range hit from a 265 might be a concern.
Well... you did say "Performance and aesthetics are my main concern", so that's the prescription I gave for your case.

You can also 255/40/18 if you don't mind -1.1% smaller diameter. You're right though, it will barely make a difference on the speedometer. More of a visual difference as it increases wheel gap.
 
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TacoCarl

Member
Mar 24, 2021
5
1
Boston, MA USA
Well... you did say "Performance and aesthetics are my main concern", so that's the prescription I gave for your case.

You can also 255/40/18 if you don't mind -1.1% smaller diameter. You're right though, it will barely make a difference on the speedometer. More of a visual difference as it increases wheel gap.
No, thank you for the reply! I appreciate all feedback.
You think acceleration is negatively affected much from a 0.4” OD difference?
 
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GregD60

Member
Mar 24, 2016
357
349
Colorado
I've been running 245/45-18 for my winter tires for 3 years now. When I replace my summer tires, probably sometime this summer, I plan to go to 245/45-18 for them as well. The slightly wider tire helps to protect the rim while not being overly wide for it. Michelin recommends at least a 9" rim for 265/40-18.
 
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MODEL+

Vendor
Oct 21, 2020
749
587
Portland, OR
No, thank you for the reply! I appreciate all feedback.
You think acceleration is negatively affected much from a 0.4” OD difference?
Nah, I don't think you'll feel anything different on the "butt dyno" with that little amount of difference in diameter. Your speedo will read 1 mph slower @ 90mph haha
 
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GregD60

Member
Mar 24, 2016
357
349
Colorado
Between being a little taller and a little wider, the 245/45-18 tires fill in the wheel wells more which I think looks better.

Also, on TireRack, the 245/45 is about $15 a tire cheaper than the 235/45 or the 255/40.
 
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TacoCarl

Member
Mar 24, 2021
5
1
Boston, MA USA
Nah, I don't think you'll feel anything different on the "butt dyno" with that little amount of difference in diameter. Your speedo will read 1 mph slower @ 90mph haha
I do value my good ole butt dyno, strangely accurate lol.
Yeah, I was concerned as well with just the idea of adding 2lbs per corner of unsprung weight and the slightly larger OD. Luckily the new wheels I bought are lighter than the oem aeros; at this point instead of trying to gain range (performance aside), I think I’m just trying to negate negative range hits. Stickier rubber, slightly wider tire, not great for that but whatever I guess, I avoided the temptation of wider wheels which I wanted at first. That would’ve meant much wider/heavier/more$ tires and a noticeable range hit, for a trade off of benefits I’d only really see on the track.
I think 245/45 will end up being a good size 👌🏻
 
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Sam1

Member
Sep 11, 2019
954
866
NV
No, thank you for the reply! I appreciate all feedback.
You think acceleration is negatively affected much from a 0.4” OD difference?

You will see zero acceleration differences because the motors' torque is electronically limited, and it's not even close to being maxed out. I removed over 40 pounds of mass on the brakes and wheels, moved to a stickier rubber with the light weight, and it wasn't even 0.01 faster on 0-60mph logged with dragy, literally the same exact times. When you accelerate while at highway speeds, it does 'feel' like the initial hit is harder (probably from the reduced weight, but my 60-130mph times are also the same exact times as they were with the OEM setup.

If you're looking for contact patch, the PS4S in 18/245/45 non-T0 has a tread width of 8.1". If you compare that to the 18/245/45 PZ4, that tire has a tread width of 7.6" (same as its 18/235/18 model) Or the 18/245/45 Ecsta PS91 has a tread width down to 7.4" ... that's 3/4 of an inch thinner than the PS4S. I used to think tire width measurements were how wide the tread was until someone on here corrected me, tread width has nothing to do with the profile width spec, so make sure you look at that.
 
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