[P1Fanatic]

Had my front tyres on my X3 replaced at about 24k miles last August. Rears still had a bit of tread after 34kmiles but noticed some hairline cracks / perishing so got them replaced last week ahead of my MOT today (statutory annual vehicle inspection here in the UK). Whilst in for the MOT & service the dealer has said that as the rears are 7.5mm and the fronts 3.5 & 3.8mm that this difference can cause issues with the Xdrive transmission so they recommend replacing the fronts. I have heard this before with 4WD/AWD as used to be a similar thing mentioned with my old Subaru Impreza.

I’ve left it for now as just seems like the usual stealer attempt to get money out of you when it’s not necessary. I will check front tyre depths tonight as suspect they probably take the edges to make it sound more urgent. Was just after people’s opinions on this as would think the xdrive system is not that fragile. I’d be surprised if the fronts are that worn in 11k miles as isn’t x-drive supposed to be predominantly rear wheel driven until it detects traction issues and sends power to the fronts? I know the fronts are taking more load under braking and cornering but a 10K mile difference in wear is quite a lot.

Thanks
Simon

[MichiganMike]

Too much difference in tire diameter may put stress on the AWD components and cause additional wear. How much difference in tire size is acceptable is a topic of considerable debate. The BMW x-Drive system is reported to tolerate greater differences in tire size than some AWD systems. Some suggest a difference up to 1% in tires is acceptable for the BMW x-Drive. See links below for some related discussions.

https://www.tirerack.com/tires/tiret....jsp?techid=18

http://x3.xbimmers.com/forums/showth...=replace+tires

http://x3.xbimmers.com/forums/showth...=replace+tires

You did not state your tire size, so I am unable to make the calculation of the exact differences in your tire sizes.

[Peter_R]

I asked this question of my dealer last year after a puncture on one of my runflats. Eventually I replaced all with NON Runflats but he (dealer) advised that there should not be problem with the F25. It is/was the E83 X3 that had a problem that is popularly called transmission bunching. I always thought that problem was due one tyre being of a different rolling thread depth and not 2 on the same axle.

Let's face it, if your dealer was completely correct with his statement, they would be making a small fortune in partly used F25 tyres and also new sets of tyres.

BMW UK advise changing tyres when they are down to 3mm and not the MOT requirement of 1.6mm.

Suggest you ring Black Circles (owned by Michelin) and ask them for a quote when you come to replacing and in any event ring them concerning the difference on tread depth and what your dealer has stated.

[P1Fanatic]

Originally Posted by MichiganMike

Too much difference in tire diameter may put stress on the AWD components and cause additional wear. How much difference in tire size is acceptable is a topic of considerable debate. The BMW x-Drive system is reported to tolerate greater differences in tire size than some AWD systems. Some suggest a difference up to 1% in tires is acceptable for the BMW x-Drive. See links below for some related discussions.

https://www.tirerack.com/tires/tiret....jsp?techid=18

http://x3.xbimmers.com/forums/showth...=replace+tires

http://x3.xbimmers.com/forums/showth...=replace+tires

You did not state your tire size, so I am unable to make the calculation of the exact differences in your tire sizes.

I have the 19’s staggered so 245/45 front and 275/40 rear.

I assume 1% is a typo?

[P1Fanatic]

Originally Posted by Peter_R

I asked this question of my dealer last year after a puncture on one of my runflats. Eventually I replaced all with NON Runflats but he (dealer) advised that there should not be problem with the F25. It is/was the E83 X3 that had a problem that is popularly called transmission bunching. I always thought that problem was due one tyre being of a different rolling thread depth and not 2 on the same axle.

Let's face it, if your dealer was completely correct with his statement, they would be making a small fortune in partly used F25 tyres and also new sets of tyres.

BMW UK advise changing tyres when they are down to 3mm and not the MOT requirement of 1.6mm.

Suggest you ring Black Circles (owned by Michelin) and ask them for a quote when you come to replacing and in any event ring them concerning the difference on tread depth and what your dealer has stated.

You are not wrong - there would be an entire industry dedicated to transmission repairs. My tyre fitter (Elite Wheels in Reading) never mentioned it being an issue and I trust their judgement on anything tyre related. They are also cheaper than black circles for the Pirelli PZero run flats.

I take everything main dealers say with a large pinch of salt. Especially when last year they recommended my rear discs need skimming and a year and 11k miles later they are now fine

[CeeTeeX5]

I've been researching this quite a bit lately as I'm trying to find tires for a set of 22s I recently purchased. The general guidance commonly provided is to stay within 1% variance in tire circumference. Some people are running a greater difference than 1% with no issues.

As a reference, the BMW recommended tire size for 21" OEM wheels for the X5 is 285/35/21 and 325/30/21, which has a .7% variance. This does lead one to believe that there is an acceptable amount of variance in tire sizes that will not result in any transfer case issues.

There is no way that tires will ever be the exact same circumference even of the same model and size so there has to be some built in play. How much play is the question. As far as I can tell no one has been able to get a straight answer from BMW on this. You'll have to decide for yourself how much risk you want take.

Hope it's ok to post a link to another forum, but there is some good discussion on this thread regarding this topic: http://www.bimmerfest.com/forums/sho...d.php?t=919371

[MichiganMike]

Originally Posted by P1Fanatic

I have the 19’s staggered so 245/45 front and 275/40 rear.

I assume 1% is a typo?

The nominal circumference difference between your front and rear tires is just under 1%. The 1% in my post was not a typo.

The link below provides a tool to calculate the tire circumference.

http://www.csgnetwork.com/tiresizescalc.html