changing both tyres on rear axle

[alziebob]

Hi,

I've just had a conversation with a guy who reckons you should change both tyres on an axle because changing just 1 tyre can affect the diff on that axle!

Is this true or is he just talking rubbish?

Thanks in advance for any helpful comments

[X5Sport]

Correct. In fact if the tread depth difference to the other axle is more than 1.5 to 2mm you need to change those as well. You should also really use tyres with a ‘*’ mark on the sidewall (means BMW Approved) or ones with MOE as BMW accept those as well. They have matched rolling radii.

The problem is that xDrive cars (as also found with other performance AWD system such as Audi, Mercedes and others) are intolerant of bad tyre choices particularly where to sizes on the axles are different - so called ‘staggered’ - and this means different rolling radii. If the two differences are more than a certain amount then damage to the transmission (broken diff or transfer box) becomes an expensive reality. Sadly this can mean having to replace a complete set when one tyre gets a puncture and can’t be repaired, and when this last happened on my X6 the bill was almost £900 for the right type of tyre.

[alziebob]

Thanks for the reply....I thought the guy that told me was just talking rubbish

[sapphireblack]

It's an idiosyncracy of BMWs X drive system and can indeed be costly.

Hopefully as a new memeber you'll stick around.

[alziebob]

thanks for your reply.....I'm not a completely new member I just dont come on very often! I just wonder how critical is the tread depth between axles?
I'm not new to BMW's I also have a 2003 E46 330ci convertible

[X5Sport]

No more than 2mm between any one tyre or axle and the rest or other axle. 1.5mm is better. The upper limit comes from the BMW Technical Instructions. See below (and applies to all AWD versions regardless of tyre size).

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[Leslie]

The cheapest option would be to get a used tyre same brand with matching tread depth unless the others are nearly new already.
The diffs are designed to accommodate large differences in speed otherwise you could not go around a tight cormer the transfer box however doesn't like it.

[alziebob]

Thanks for your helpful comments much appreciated