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      12-07-2018, 10:54 AM   #1
Miko M
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I want to end the guessing game and worries for AWD differential being damaged.

I know this is gray area for most people, here is a simple calculation for Circumference:
( people that are not good in calculating or want easier and quick calculations like plug and play see bottom )

Circumference = 2 π or
Circumference = π D

I am assuming everyone knows π =3.14159

I would think most people know how to calculate the % so we need to stay within 1.3% for AWD to protect differential!!!!

I have seen people do up to 1.8% or more but this is up to individual how much they want to risk.

To make it easier on everybody, I am attaching this tire size comparison which is basically: plug and play

There is so much more info that everyone can use.

I am sure most people will appreciate this.


https://tiresize.com/comparison/
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      01-23-2019, 12:45 AM   #2
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What do you think about 295/30/R21 and 315/30/R21?

It's 1.4% but you can get Pilot Sport 4S
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      01-23-2019, 03:47 AM   #3
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So are you saying there should be a max difference of 1.3% in total between the front and rear?

Or a 1.3% difference from the OEM difference?

(Given that the OEM tires are staggered and of slightly different diameters)
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      01-23-2019, 05:37 AM   #4
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Quote:
Originally Posted by Beck. View Post
What do you think about 295/30/R21 and 315/30/R21?

It's 1.4% but you can get Pilot Sport 4S
Quote:
Originally Posted by Viper7 View Post
So are you saying there should be a max difference of 1.3% in total between the front and rear?

Or a 1.3% difference from the OEM difference?

(Given that the OEM tires are staggered and of slightly different diameters)
Normally 1.3% from stock
I talked to few rims manufacturers and looks like they like to stay close to OEM .
I have seen some guys run 275/30 and 315/25 -1.1% you not supposed to run negativ on the back but people do

Or 265/30 and 325/25 +.4%

I would recommend to check with manufacturer sense they are the one responsible for selling.
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      01-23-2019, 12:02 PM   #5
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So the OEM difference is 1.1% already right?

Does that mean you can safely deviate another 1.3% away from that?
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      01-23-2019, 12:32 PM   #6
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Quote:
Originally Posted by Viper7 View Post
So the OEM difference is 1.1% already right?

Does that mean you can safely deviate another 1.3% away from that?
Yes stock is 1.1%
I believe differential are set up for 1.1% already
So if you stay within 1.3% from stock you should be fine.
If you ask bmw they wouldn't recommend anything but stock. Lol
It's weird how Bmw only knows the numbers that computer shows. Most likely wouldn't recommend anything else but stock tires.
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      01-24-2019, 07:23 PM   #7
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Quote:
Originally Posted by Miko M View Post
Yes stock is 1.1%
I believe differential are set up for 1.1% already
So if you stay within 1.3% from stock you should be fine.
If you ask bmw they wouldn't recommend anything but stock. Lol
It's weird how Bmw only knows the numbers that computer shows. Most likely wouldn't recommend anything else but stock tires.
If I bought 20” I was going to try R888R in 285/35 front and 305/35 rear. This makes me reconsider since it is 1.8% difference.

Think I will get 21” and roll 285/30 front and 295/30 rear at 1.1% to match stock
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      01-24-2019, 07:30 PM   #8
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Quote:
Originally Posted by apsuard View Post
Quote:
Originally Posted by Miko M View Post
Yes stock is 1.1%
I believe differential are set up for 1.1% already
So if you stay within 1.3% from stock you should be fine.
If you ask bmw they wouldn't recommend anything but stock. Lol
It's weird how Bmw only knows the numbers that computer shows. Most likely wouldn't recommend anything else but stock tires.
If I bought 20" I was going to try R888R in 285/35 front and 305/35 rear. This makes me reconsider since it is 1.8% difference.

Think I will get 21" and roll 285/30 front and 295/30 rear at 1.1% to match stock
Most wheel manufacturers recommend 285/35/20 and 305/35/20
I ordered wheels that are coming next week
10" front and 11" back
Will go most likely with that tire size above.
I was thinking to go with R888R too but, friend of mine had it and said you will pick everything from the road and have more damage on side off the car from the stone/ sand
So I might go with different brand
Rear tires will wear quicker so it can be higher
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      01-24-2019, 11:21 PM   #9
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Quote:
Originally Posted by apsuard View Post
Quote:
Originally Posted by Miko M View Post
Yes stock is 1.1%
I believe differential are set up for 1.1% already
So if you stay within 1.3% from stock you should be fine.
If you ask bmw they wouldn't recommend anything but stock. Lol
It's weird how Bmw only knows the numbers that computer shows. Most likely wouldn't recommend anything else but stock tires.
If I bought 20” I was going to try R888R in 285/35 front and 305/35 rear. This makes me reconsider since it is 1.8% difference.

Think I will get 21” and roll 285/30 front and 295/30 rear at 1.1% to match stock
I think this expert is telling us the car is set up for a 1.1% difference anyway, so it's the deviation from that which is important.

So a 1.8% difference overall isn't actually much of a difference from OEM, if you know what I mean.

At least that's my interpretation of the original comments...
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      01-25-2019, 12:26 AM   #10
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A few things I'd like to point out.

The F8x M3 and M4 use taller tyres at the rear in every version of about the same difference front to rear. When people change the ratio closer to being the same size or even run shorter tyres in the rear, the traction control cuts in much more frequently.

The F90 M5 can have 1.1% difference if you have the standard summer tyres on. However on winter tyres you are able to do 265/40r19 all round, so no difference at all as well as 265/40r19 at the front and 285/40r19 at the rear. This is 2.6% difference.

As such I believe it will be fine if you go anywhere from 0% to 2.6% difference.

It is also worth noting that the same system in staggered wheels on the G30, G32 and G11 have slightly shorter wheels at the rear, up to 1.1% shorter at the rear. As the overall system design is the same and isn't a constant 50/50 setup which would bind (normally the front wheels are disengaged) that slightly shorter at the rear would also be ok. However I wouldn't do this for the reason I mentioned above, the traction and stability systems are programmed for slightly taller tyres at the rear.
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      01-25-2019, 05:01 AM   #11
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Quote:
Originally Posted by brad850csi View Post
A few things I'd like to point out.

The F8x M3 and M4 use taller tyres at the rear in every version of about the same difference front to rear. When people change the ratio closer to being the same size or even run shorter tyres in the rear, the traction control cuts in much more frequently.

The F90 M5 can have 1.1% difference if you have the standard summer tyres on. However on winter tyres you are able to do 265/40r19 all round, so no difference at all as well as 265/40r19 at the front and 285/40r19 at the rear. This is 2.6% difference.

As such I believe it will be fine if you go anywhere from 0% to 2.6% difference.

It is also worth noting that the same system in staggered wheels on the G30, G32 and G11 have slightly shorter wheels at the rear, up to 1.1% shorter at the rear. As the overall system design is the same and isn't a constant 50/50 setup which would bind (normally the front wheels are disengaged) that slightly shorter at the rear would also be ok. However I wouldn't do this for the reason I mentioned above, the traction and stability systems are programmed for slightly taller tyres at the rear.
Agree with you on that one
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      01-25-2019, 12:53 PM   #12
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So in short - most of the commonly used tire sizes that enthusiasts opt for when changing wheels are probably well within the margin of error, and will likely work just fine.

Which I guess is the reason I've never seen any negative feedback on here about slightly different tire sizes from those who have upgraded.
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      01-25-2019, 07:31 PM   #13
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Quote:
Originally Posted by Viper7 View Post
So in short - most of the commonly used tire sizes that enthusiasts opt for when changing wheels are probably well within the margin of error, and will likely work just fine.

Which I guess is the reason I've never seen any negative feedback on here about slightly different tire sizes from those who have upgraded.
To be hones negative feed back might come in later time. It might take time to have effect on differential
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