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      03-03-2020, 09:56 AM   #1
GSG5
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Impact on Speedometer?

Would changing the total diameter of the rear tires only (not front) impact the accuracy of the speedometer? I've read that the speedometer generally measures wheel revolutions via the transmission which means that its effectively linked to the drive wheels - so the rears for a RWD or the fronts for a FWD. I can't find anything definitive on and AWD transmission like the F95.

Depending on just how harsh the ride is on the F95, I'm considering increasing the profile slightly on the rears only (going from 30 to 35). Based on the pictures, it appears that there is plenty of space available in the wheel well for a slightly thicker profile tire. I may wait until I actually need new rear tires to make the change, but just thinking ahead.

Maybe I'm over thinking this ride issue, but just curious if such a change to only the rears would throw off the speedometer or if its tied to the fronts instead.

Thoughts?
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      03-03-2020, 10:20 AM   #2
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Have you a rwd or awd?
If rwd you might be ok to do it, not sure how it'll affect your speedo though or possibly traction control and other safety features.
On awd you want to keep front and rear diameters the same or as close as possible. I'm not sure the tolerance but changes outside a certain percent can cause failure/issues and likely void warranty.
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      03-03-2020, 10:22 AM   #3
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https://m.tirerack.com/tires/tiretec....jsp?techid=18
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      03-03-2020, 11:39 AM   #4
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2008 BMW M3 E92  [4.39]
I have 20s on an E92 M3. Between the tire and wheel OEM differences, my speedometer is off by about 2 MPH.
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      04-03-2020, 03:19 PM   #5
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I just changed the tires on my F95 to 295/40 front and 315/35 rear from 295/35 and 315/30. Same 21 and 22 inch PZ4's so just a bit more height. I did this for two reasons: (1) so that there is less empty space in the wheel wells and (2) a bit more of a comfortable ride. A little less road feel but a smoother ride. I'm very pleased with the outcome.

Last edited by GlennC; 04-05-2020 at 08:59 AM..
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      04-03-2020, 03:24 PM   #6
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Quote:
Originally Posted by GlennC View Post
I just changed the tires on my F95 to 275/45 front and 315/35 rear from 275/40 and 315/30. Same 21 and 22 inch PZ4's so just a bit more height. I did this for two reasons: (1) so that there is less empty space in the wheel wells and (2) a bit more of a comfortable ride. A little less road feel but a smoother ride. I'm very pleased with the outcome.
This is EXACTLY what I was/am planning to do... but once I actually own one! Same exact reasoning - as much to fill the negative space as to improve the ride.
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      04-03-2020, 08:28 PM   #7
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Quote:
Originally Posted by GSG5 View Post
Quote:
Originally Posted by GlennC View Post
I just changed the tires on my F95 to 275/45 front and 315/35 rear from 275/40 and 315/30. Same 21 and 22 inch PZ4's so just a bit more height. I did this for two reasons: (1) so that there is less empty space in the wheel wells and (2) a bit more of a comfortable ride. A little less road feel but a smoother ride. I'm very pleased with the outcome.
This is EXACTLY what I was/am planning to do... but once I actually own one! Same exact reasoning - as much to fill the negative space as to improve the ride.
Note: I corrected my post above new front tires are 295/40 (from 295/35).
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      04-09-2020, 04:23 PM   #8
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Quote:
Originally Posted by GlennC View Post
I just changed the tires on my F95 to 295/40 front and 315/35 rear from 295/35 and 315/30. Same 21 and 22 inch PZ4's so just a bit more height. I did this for two reasons: (1) so that there is less empty space in the wheel wells and (2) a bit more of a comfortable ride. A little less road feel but a smoother ride. I'm very pleased with the outcome.
Hi GlennC,

I donít have my car yet, but Iím really interested in following suit with the tire size swap. Did you get the same exact tire brand/model? I was told by Discount Tire that neither of the stock tire brands/models for the F95 (Michelin 4s or Perrelli PZero) come in a 21/295/40 or a 22/315/35. Perhaps heís just wrong, but was curious if you needed to change tire models?

Thanks
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      04-09-2020, 05:34 PM   #9
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Quote:
Originally Posted by GSG5 View Post
Hi GlennC,

I donít have my car yet, but Iím really interested in following suit with the tire size swap. Did you get the same exact tire brand/model? I was told by Discount Tire that neither of the stock tire brands/models for the F95 (Michelin 4s or Perrelli PZero) come in a 21/295/40 or a 22/315/35. Perhaps heís just wrong, but was curious if you needed to change tire models?

Thanks
Hi GSG5,
He's wrong. You can see for yourself at tirerack.com. The Pirelli P Zero's come in 21/295/40 and 22/315/35 and they are on my car for a week now. I had them shipped to my house and then had Pep Boys swap them out. I'm very pleased I did it. Less road feel as I mentioned which is a bit of a negative but smoother ride and less pothole anxiety.
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      04-09-2020, 09:14 PM   #10
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Quote:
Originally Posted by GlennC View Post
Hi GSG5,
He's wrong. You can see for yourself at tirerack.com. The Pirelli P Zero's come in 21/295/40 and 22/315/35 and they are on my car for a week now. I had them shipped to my house and then had Pep Boys swap them out. I'm very pleased I did it. Less road feel as I mentioned which is a bit of a negative but smoother ride and less pothole anxiety.
Great, thanks. I figured that may be the case - that he is just wrong.

Glad to hear youíre happy with the switch. Honestly, Iím as interested in filling the negative space as improving the ride. The more pics I see of F95s on dealer sites around the country, the more I notice the extra space above the tire in the wheel well. I donít plan to track mine so Iíd prefer to have a bit more rubber (which also mitigates the blow out risk too).
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      04-10-2020, 09:04 AM   #11
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Excuse the dirt but here are photos with the larger tires.
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      04-10-2020, 10:04 AM   #12
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Thanks for the pics. Looks great, man...an improvement IMO.
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