Tire size question

Lviv777 · 01-02-2019, 02:46 AM

On my '11 I am running 255/40/21 fronts and 295/35/21 rears. I been reading about the 1% difference to not screw up the transfer case. Is my size currently fine or should I change it out? Thank you! Sorry if this has been asked before.

Thecastle · 01-02-2019, 03:37 AM

I’m not sure where you came up with 1% tolerance, since the official tolerance isn’t documented anywhere...

Anyway look up your tires and check the revolutions per mile for the specific size and that will tell you how close they are. Some difference is completely normal and not an issue.

1gokart · 01-02-2019, 04:37 PM

Stock BMW X5 is 255/55 R18 which is a 737 OD.
Staggered 19" are 255/50 R 19 Front and 285/45 R19 which are both 739 OD.
Staggered 20" are 275/40 R20 Front and 315/35 R20 which are both 728 OD.

Your setup of 255/40 R21 is a 737 OD and the 295/35 R21 is a 739 OD. The difference is 2mm, is that the 1% deviation you're speaking of?

I'm assuming the question you're asking is whether the 2mm difference in tire OD from front to rear have an impact to the differential or 4WD system?

I would think the tolerance is acceptable but I'm not a tire/wheel engineer. FYI, TireRack.com advertise a different 21" setup with a higher 4mm difference from front to rear for the BMW X5M configuration.

TwinSnailz · 01-02-2019, 05:09 PM

Fun fact: Did you know EVERYTIME you turn, the outer wheel is spinning more revolutions. This is where a differential serves it purpose. If you’re afraid of the 1% difference remove your steering wheel.

Lviv777 · 01-02-2019, 07:26 PM

Quote:

Originally Posted by TwinSnailz

Fun fact: Did you know EVERYTIME you turn, the outer wheel is spinning more revolutions. This is where a differential serves it purpose. If you’re afraid of the 1% difference remove your steering wheel.

I meant transfer case

Lviv777 · 01-02-2019, 07:27 PM

My mistake, I meant to say the transfer case, not differential.

01-02-2019, 02:46 AM	#1
Lviv777 Second Lieutenant 42 Rep 269 Posts Drives: f30 330e, C8 Vette Join Date: Oct 2017 Location: Highway iTrader: (0)	Tire size question On my '11 I am running 255/40/21 fronts and 295/35/21 rears. I been reading about the 1% difference to not screw up the transfer case. Is my size currently fine or should I change it out? Thank you! Sorry if this has been asked before. Last edited by Lviv777; 01-02-2019 at 07:26 PM..
	Appreciate 0 Tweet Quote

01-02-2019, 03:37 AM	#2
Thecastle Major 307 Rep 1,146 Posts Drives: F10 550i Join Date: Apr 2015 Location: Houston, TX iTrader: (0) Garage List 2020 BMW M340i [0.00] 2019 RAM 3500 [0.00] 2020 BMW X3 [0.00] 2016 BMW 550i [0.00] 2000 Ford F450 7.3l ... [0.00]	I’m not sure where you came up with 1% tolerance, since the official tolerance isn’t documented anywhere... Anyway look up your tires and check the revolutions per mile for the specific size and that will tell you how close they are. Some difference is completely normal and not an issue.
	Appreciate 0 Quote

01-02-2019, 04:37 PM	#3
1gokart New Member 0 Rep 8 Posts Drives: 2013 X5 50i M-Sport Join Date: Dec 2018 Location: MI iTrader: (0)	Stock BMW X5 is 255/55 R18 which is a 737 OD. Staggered 19" are 255/50 R 19 Front and 285/45 R19 which are both 739 OD. Staggered 20" are 275/40 R20 Front and 315/35 R20 which are both 728 OD. Your setup of 255/40 R21 is a 737 OD and the 295/35 R21 is a 739 OD. The difference is 2mm, is that the 1% deviation you're speaking of? I'm assuming the question you're asking is whether the 2mm difference in tire OD from front to rear have an impact to the differential or 4WD system? I would think the tolerance is acceptable but I'm not a tire/wheel engineer. FYI, TireRack.com advertise a different 21" setup with a higher 4mm difference from front to rear for the BMW X5M configuration.
	Appreciate 0 Quote

01-02-2019, 05:09 PM	#4
TwinSnailz First Lieutenant 75 Rep 357 Posts Drives: 2011 X5//M Join Date: Nov 2009 Location: IL iTrader: (0)	Fun fact: Did you know EVERYTIME you turn, the outer wheel is spinning more revolutions. This is where a differential serves it purpose. If you’re afraid of the 1% difference remove your steering wheel.
	Appreciate 0 Quote

01-02-2019, 07:27 PM	#6
Lviv777 Second Lieutenant 42 Rep 269 Posts Drives: f30 330e, C8 Vette Join Date: Oct 2017 Location: Highway iTrader: (0)	My mistake, I meant to say the transfer case, not differential.
	Appreciate 0 Quote