[Pacific747]

Anyone know what type of differential machism is inside the transfer box? I’m curious as I understand that changing tyre sizes form oem can cause damage to transfer box . This makes little sense to me as oem wheels on my x5 are 2.7% different in size

[Tejas1836]

Diagram, parts list ATC13, yours may be a little different
https://awd.tech/apps/catalog/transmission/atc13-bmw

[Ninefourteener]

Originally Posted by Pacific747

Anyone know what type of differential machism is inside the transfer box? I’m curious as I understand that changing tyre sizes form oem can cause damage to transfer box . This makes little sense to me as oem wheels on my x5 are 2.7% different in size

OEM tires are less than 1% difference in circumference, whether you have 20s, 21s, or 22s.

X5 transfer cases are a known point of failure, with a lot of controversy surrounding it. V8 X5s start showing symptoms usually around 60k miles, and I-6 X5s usually around 10k later. BMW says the fluid is "lifetime" fluid..... and it isn't.

Almost every expert (except BMW) says you should change the fluid every 50k miles.

I was lucky...I started showing signs of slipping at about 58K miles, changed the fluid immediately.... and the issue went away 100%.

If you wait too long.... your xfer case is toast.

The OEM fluid is expensive (about $40)... but you only need 1 bottle, and the process was MUCH easier than I expected. I plan to do it every 20k miles from this point forward. It's too easy - might as well.

[Pacific747]

My oem tyres have a 2.7% difference, 315 30 22 vs 275 35 22. I assume that the transfer is designed to cope with this difference. I’m planning on going to,325 30 22 on rear , this would make front and rear same height in theory. Anyone tried this size combo ?

[Ninefourteener]

The sidewall height 3.8 vs 3.7..... one-tenth of an inch. That's where you are getting your 2.6% difference.

The difference in rolling circumference (which is what matters) is nearly identical, with only a 0.7% difference.

That's why BMW uses them.

[weinerbarn]

Originally Posted by Ninefourteener

OEM tires are less than 1% difference in circumference, whether you have 20s, 21s, or 22s.

X5 transfer cases are a known point of failure, with a lot of controversy surrounding it. V8 X5s start showing symptoms usually around 60k miles, and I-6 X5s usually around 10k later. BMW says the fluid is "lifetime" fluid..... and it isn't.

Almost every expert (except BMW) says you should change the fluid every 50k miles.

I was lucky...I started showing signs of slipping at about 58K miles, changed the fluid immediately.... and the issue went away 100%.

If you wait too long.... your xfer case is toast.

The OEM fluid is expensive (about $40)... but you only need 1 bottle, and the process was MUCH easier than I expected. I plan to do it every 20k miles from this point forward. It's too easy - might as well.

Agree it should be changed more often but also keep in mind BMW admitted to filling the wrong fluid on some years. I suspect many of the failures are rearing their ugly head from that.

[magna6882]

my 2019 x5 50i had failure at 30000 miles.I think the advise at swapping the fluid is prudent especially with it being so easy. My transfer case noise really became apparent with new tires. I thought it was the tires so had them swap them with another set but no change.If i had heard about the transfer case fluid problem i likely could have avoided the replacement .

[bouchy]

There isn't a differential (nor gears), it's a chain, unfortunately.
It looks like this:

Even though when google searching it, it shows a lot of BS in first 100 answers: https://www.google.com/search?sca_es...&bih=912&dpr=1

Change the fluid regularly (for what P/N?).

[bloom]

83-22-2-409-710 DTF1 fluid 1liter

Buy from dealership.. $82CAD

Research procedure how to change it before attempting.