Different tyres sizes?????

JonbmwX5 · #1 10-01-2012, 10:23 AM

Hi guys, i have just realise after all this time i have been running my x5 on two different size tyres

the two at the front are different to the two at the back. i have 295 30 22 and 285 35 22. Im worried now that it might have damaged the trans, do i need to replace two asap or is this ok untill i get paid (a few weeks). A friend says two need to be replaced asap as it will break the transmission. Another said it should be fine as it a 4x4 and they supply different power to different wheels.

i did a google search and it says different things about "the rolling radius would be different"

i dont really get the rolling radius thing, does 5mm make that much difference, are they ment to be different (like race cars)

i would be very gratefull of any advice, many thanks kind regards Jon

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TwinsPoppa · #2 10-01-2012, 10:50 AM

A couple things to consider.

Using rolling diameters different than stock will affect the speedometer - reading higher or lower than actual.

Driveline damage, as it relates to using different rolling diameters front to back, are dependent on your model X5.

2003 and below (non-xdrive) have a locked front/back torque split. If your X does not have xdrive then match your front & rear tires. If I remember correctly they can only be within 3-4 revs/mile tolerance difference.

If you use a site like this Tire Size Calculator, you can compare tire specs. There are differences between mfgrs even with the same sized tires. So, while the dimensions are nominal, and not actual, they are fine for our purposes.
Tire Size Calculator - tire & wheel plus sizing?

According to that site your current setup has about 21 revs/mile difference.

2004 models and up have x-drive. These allow for more tolerance as the torque split is not locked. However, I can't remember right now what is the tolerance.

What year/model is your X?

JCL · #3 10-01-2012, 03:41 PM

Twins:

A couple of comments.

You say that the pre x-drive has a locked torque split. That is technically true, but a little misleading IMO. The transfer case has a fixed torque split, but it is certainly not locked. The transfer case is open, and one drive shaft (front or rear) can rotate at a different speed than the other without too much issue. If you have a very large difference, for a long time, then damage can result. Towing with one axle raised and the other axle on the ground is an example; you are limited to a certain speed and distance by BMW.

Tire size differences can certainly cause damage with the pre x-drive transfer case. I would strive for 3-4 revolution per mile difference, but I don't think that is a hard cutoff. When BMW came out with the x-drive transfer case (in the X3) there were actually more problems with tire size differences. X3 models are famous for it, as they have a lighter transfer case than the E53. So I would be just as concerned with an x-drive transfer case and tire size differences, including on the E53/E70, probably more so than with the pre x-drive.

TwinsPoppa · #4 10-01-2012, 04:56 PM

Quote:

Originally Posted by JCL

Twins:

A couple of comments.

You say that the pre x-drive has a locked torque split. That is technically true, but a little misleading IMO. The transfer case has a fixed torque split, but it is certainly not locked. The transfer case is open, and one drive shaft (front or rear) can rotate at a different speed than the other without too much issue. If you have a very large difference, for a long time, then damage can result. Towing with one axle raised and the other axle on the ground is an example; you are limited to a certain speed and distance by BMW.

Tire size differences can certainly cause damage with the pre x-drive transfer case. I would strive for 3-4 revolution per mile difference, but I don't think that is a hard cutoff. When BMW came out with the x-drive transfer case (in the X3) there were actually more problems with tire size differences. X3 models are famous for it, as they have a lighter transfer case than the E53. So I would be just as concerned with an x-drive transfer case and tire size differences, including on the E53/E70, probably more so than with the pre x-drive.

Correction, the non-xdrive transfer case has a fixed front-to-rear torque split.

So, in general and all things being equal, you feel the non-xdrive transfer case would fare better when it comes to using mismatched rolling diameters in the front vs. rear? I'm probably missing something and don't know it.

JonbmwX5 · #5 10-01-2012, 05:02 PM

thank you for your replys, wow i didnt realise different tyre sizes made so much difference, my other cars a alfa spider and i have never had to look into this much info about tyre sizes, but hey its always broke down lol!........... (not really)

my x5 is a 2001 e53 3.0l petrol so its not a xdrive so not looking good, im going to have a play on the website now to see what it come up with................ps nice cars

TwinsPoppa · #6 10-01-2012, 05:09 PM

Quote:

Originally Posted by JonbmwX5

thank you for your replys, wow i didnt realise different tyre sizes made so much difference, my other cars a alfa spider and i have never had to look into this much info about tyre sizes, but hey its always broke down lol!........... (not really)

my x5 is a 2001 e53 3.0l petrol so its not a xdrive so not looking good, im going to have a play on the website now to see what it come up with................ps nice cars

You should run all the same sized rolling diameters. Maybe just run the 285 on all four corners - as long as they are the same rolling diameter.

Stock some X5s came with a staggered setup of 275/40-20 front and 315/35-20 tires. However, they still had the same rolling diameter.

If you want to run staggered wheels and/or tires you can - just make sure they are the same rolling diameters.

Thanks for the compliment.

JonbmwX5 · #7 10-01-2012, 05:21 PM

ok so i used the website and its say the rev per miles on the 295/30/22 are 718
and the 285/35/22 are 697 so there is a difference.

the rolling radius for the 295 is 14.48 inch
and the radius for the 285 is 14.92 inch

so for future use if i go for different tyre brands, whats the variance you can have?

TwinsPoppa · #8 10-01-2012, 05:28 PM

Quote:

Originally Posted by JonbmwX5

ok so i used the website and its say the rev per miles on the 295/30/22 are 718
and the 285/35/22 are 697 so there is a difference.

the rolling radius for the 295 is 14.48 inch
and the radius for the 285 is 14.92 inch

so for future use if i go for different tyre brands, whats the variance you can have?

Ideally you want the same but try to be within 3-4 revs/mile max. Try to have the same tire brands too. Why would you want to use different tire brands?

JonbmwX5 · #9 10-01-2012, 05:36 PM

the ones on the X are nexens, not a brand i know, i will replace two tyres with a better brand and then replace the nexens at a later date. thats the plan lol!

JCL · #10 10-01-2012, 10:40 PM

Quote:

Originally Posted by TwinsPoppa

Correction, the non-xdrive transfer case has a fixed front-to-rear torque split.

So, in general and all things being equal, you feel the non-xdrive transfer case would fare better when it comes to using mismatched rolling diameters in the front vs. rear? I'm probably missing something and don't know it.

An old pick up or Jeep has a locked up transfer case in 4wd, simply dividing the power front rear but not allowing any speed difference. That is a very different type of transfer case.

I don't have data to back it up, but yes, for the following reasons

*The non x-drive (nxd) is an open diff. It can handle speed differences
*The xd has clutches to wear
*The X3 (which only came with xd) had more problems with transfer case failures, attributed by BMW to the use of different sized tires.
*BMW has a towing limit, with one axle raised, of 150 km at 50 km/hr for the nxd. Towing the xd with one axle raised is not permitted due to the risk of transfer case damage.

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