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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Wheels and Tires Forum Sponsored by The Tire Rack > How to Adjust Wheel Size For Different Bumpers and Speedometer?



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      04-09-2019, 12:04 PM   #1
sKruPz
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How to Adjust Wheel Size For Different Bumpers and Speedometer?

Hello, I am currently running oem size wheels and installed M3 front and rear bumpers, which leave the wheel gap even larger. VMR told me that for my car (e90 330xi sedan) can run this:

Front: 19x8.5 ET35 // 235/35/19
Rear: 19x9.5 ET45 // 265/30/19

But this doesn't account for the bumpers that enlarge the wheel gap. Also, my speedometer will be off when I change wheels, so what do you recommend I do? Is that something only a dealer can help me fix? Thank you
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      04-09-2019, 12:55 PM   #2
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Uhhhmm, your wheel gap will not get any bigger because of the m3 bumpers, they will still be the same distance away from the fender. The m3 bumpers still follow the same wheel arch of your stock fenders, impossible for the gap to the wheels to get bigger.

About your speedometer, your new rim size might be bigger, but you compensate that by getting a smaller sidewall tire which keeps your overall diameter the same size as stock/previous.
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      04-09-2019, 01:36 PM   #3
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I had 19x8.5 ET +28 and 19x10 ET +30 on my 328 xi, on problem.

Tires get pretty skinny though, and I'm dodging pot holes on known bad roads with these 19".

As far as I know, the over all circumference of your new setup needs to be nearly identical to your original setup. keep circumference difference do an absolute minimal as you do have an X drive and you do not want to break your Differential. If you don't change your circumference much, then your speedo will be accurate enough. also, I don't think it's re-programmable as your speed is tied into many other carly functions.

Last edited by Pinscher; 04-09-2019 at 01:47 PM.
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      04-09-2019, 04:52 PM   #4
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Okay, thank you guys. I think by wheel gap I mean width of the wheel "area"? The bumpers have a wider arch than the OEM. I wish I had a photo of before/after, but it's definitely noticeable.
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      04-15-2019, 11:17 AM   #5
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Quote:
Originally Posted by Pinscher View Post
I had 19x8.5 ET +28 and 19x10 ET +30 on my 328 xi, on problem.

Tires get pretty skinny though, and I'm dodging pot holes on known bad roads with these 19".

As far as I know, the over all circumference of your new setup needs to be nearly identical to your original setup. keep circumference difference do an absolute minimal as you do have an X drive and you do not want to break your Differential. If you don't change your circumference much, then your speedo will be accurate enough. also, I don't think it's re-programmable as your speed is tied into many other carly functions.

What do you mean by breaking my differential? My original setup is pretty small and I'd definitely like to increase circumference, but I didn't know there would be issues
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      04-15-2019, 04:25 PM   #6
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Quote:
Originally Posted by sKruPz View Post
What do you mean by breaking my differential? My original setup is pretty small and I'd definitely like to increase circumference, but I didn't know there would be issues

Well if the rolling distance of your rear wheels is greater than the front then the rate the differentials spin at are not in sync. Sorry for lack of technical terms here, but basically your XI will not compensate for large discrepancy, and you can basically ruin your differential or transfer case.

This is why XI come with a Square wheel/tire setup, though it's possible to go staggered, provided the rolling distance of your F to R tires is within 5% or so.
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      04-15-2019, 04:35 PM   #7
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Front & rear bumpers won't increase the area under your wheel arches, unless you also have M3 fenders installed. Pics would help, super easy to upload to imgur and post the link here.

Since you have an XI, you will need to make sure your front & rear rolling diameters are within 3% of each other. The sizes you listed will be fine (647.1mm vs 641.6mm, 0.86% difference).

Assuming you are running the stock 18" tire sizes (225/40 and 255/35), you will only be increasing your rolling diameter by 1-1.5%, so your speedometer will read 1-1.5% slower which is negligible (100mph will read as 99mph..)
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      04-15-2019, 09:58 PM   #8
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Front & rear bumpers won't increase the area under your wheel arches, unless you also have M3 fenders installed. Pics would help, super easy to upload to imgur and post the link here.

Since you have an XI, you will need to make sure your front & rear rolling diameters are within 3% of each other. The sizes you listed will be fine (647.1mm vs 641.6mm, 0.86% difference).

Assuming you are running the stock 18" tire sizes (225/40 and 255/35), you will only be increasing your rolling diameter by 1-1.5%, so your speedometer will read 1-1.5% slower which is negligible (100mph will read as 99mph..)
I will take photos and upload this weekend, I am at school so I can't right now. My stock wheels are 17x8 square with 225/45R17 sized tires. The speedometer thing seems okay then!
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      04-15-2019, 09:59 PM   #9
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Quote:
Originally Posted by Pinscher View Post
Well if the rolling distance of your rear wheels is greater than the front then the rate the differentials spin at are not in sync. Sorry for lack of technical terms here, but basically your XI will not compensate for large discrepancy, and you can basically ruin your differential or transfer case.

This is why XI come with a Square wheel/tire setup, though it's possible to go staggered, provided the rolling distance of your F to R tires is within 5% or so.
I would go square if necessary, but I think I'd prefer a staggered setup. I'll keep them at around 3%
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