[APJ]

I think this has been covered before, but i can't find any of the previous threads about it . I'm sure there is a conversion chart for the xdrive system to keep your tyre sizes under 1 %. I have just had some MPS4S 235/35 20 fitted to the front wheels which are 8.5 inch rims, and can't decide whether 265/30 or 275/30 are most suitable on the on 9.5 inch rims for the rear, and which is the most ideal for the awd system. I'm hoping one of you very kind fella's out there who may have the chart, can tell me which one's are more suitable. Cheers in advance.

[robbiep]

There's a table on the driver's door pillar that tells you the sizes to use.

[Wills2]

BMW have got you covered they recommend the tyre sizes for the car...

[APJ]

Originally Posted by robbiep

There's a table on the driver's door pillar that tells you the sizes to use.

Not that simple, after several months of owning the car i realised that the 405's were replica's and also that the rim widths where wider than standard, being 8.5 and 9.5, oem are 8.0 and 8.5 (i think).
So thats why its a bit more complicated in getting the tyre sizes correct especially for awd drive setup, that so many on the forum have mentioned in the past.

[robbiep]

So your fronts are 8.5, genuine wheels are 8.0, The proper tyre size is (I believe) 225/35. Which you could easily fit on the 8.5" wide fronts.
But you've gone and bought 235/35 instead.
Once again, the 'official' sizing of 255/30 would work perfectly fine on your 9.5" wide rear wheels.

Might I suggest just going to the proper sizes, and keeping it simple, not having insurance grief in the event of an accident (or if you tell them, not having to pay additional premium).

[TiredGeek]

Have a play around on here and find some rear size that goes along with your choice of fronts but keeps within the 1% recommendation...…
https://tiresize.com/comparison/

To save you the time, 275/30 fit the bill with 0% difference in circumference

[Skyhigh]

Read this thread through: https://f30.bimmerpost.com/forums/sh....php?t=1688975

[robbiep]

Originally Posted by TiredGeek

Have a play around on here and find some rear size that goes along with your choice of fronts but keeps within the 1% recommendation...…
https://tiresize.com/comparison/

To save you the time, 275/30 fit the bill with 0% difference in circumference

But if he gets any xDrive / driveshaft / TC problems, then BMW will tell him to do one, assuming his car is still under warranty, because he isn't running the car to spec.

One hell of a way to risk a bill knocking up towards £5k, just for the sake of fitting custom tyre sizes ...

[teaston]

Originally Posted by robbiep

But if he gets any xDrive / driveshaft / TC problems, then BMW will tell him to do one, assuming his car is still under warranty, because he isn't running the car to spec.

One hell of a way to risk a bill knocking up towards £5k, just for the sake of fitting custom tyre sizes ...

But if there’s 0% difference then there’s 0% problem!

[johnung]

Originally Posted by APJ

I think this has been covered before, but i can't find any of the previous threads about it . I'm sure there is a conversion chart for the xdrive system to keep your tyre sizes under 1 %. I have just had some MPS4S 235/35 20 fitted to the front wheels which are 8.5 inch rims, and can't decide whether 265/30 or 275/30 are most suitable on the on 9.5 inch rims for the rear, and which is the most ideal for the awd system. I'm hoping one of you very kind fella's out there who may have the chart, can tell me which one's are more suitable. Cheers in advance.

There is no chart that is accurate for the simple reason that the sizes on the tire sidewall are not accurate. They are just ballparks so following them risks ending up outside the 1% specification for xDrive and costing yourself, oh about $4,000 for a new transfer case. So that means that those Internet sizing apps that guys are sure to send you are faulty too! Don't believe them!

The only way to be certain that you are within the 1% allowed differential between front and rear tires is to accurately measure the front and rear tires. Fortunately for us, TireRack measures every tire that they sell. Got to TireRack, lookup the exact tire model and exact size. Then go to the Specifications Tab and you will find the measured diameter of that exact model & size tire.

Then calculate to see if you are within 1% difference.

For example:
25.9" diameter front
Divided by
26.1" diameter rear
Equals
.9923
Times
100
Equals
99.23%

100% minus 99.23%
Equals
0.77% difference which is less than 1.0% so you would be okay if your front and rear tires had those measurements

Hope this helps!

[seenthelight!]

Fwiw, 275x30x20 on the rears are an EXACT match for the 235x35x20 on the front ... No difference in diameter at all.

[APJ]

Originally Posted by Skyhigh

Read this thread through: https://f30.bimmerpost.com/forums/sh....php?t=1688975

Exactly the reason i want to get it right, when i got the car it had 225's on the front and 275's on the back, and all the tyres were brand new, so initially in no rush to change anything, but its now eighteen months later and the car has done fifteen k in that time, then when you read various threads about transfer boxes, and i want to get it right. The 225's on the front looked awful with the rims sticking out, hence why i changed them to 235's.

[APJ]

Originally Posted by TiredGeek

Have a play around on here and find some rear size that goes along with your choice of fronts but keeps within the 1% recommendation...…
https://tiresize.com/comparison/

To save you the time, 275/30 fit the bill with 0% difference in circumference

Cheers mate, exactly what i was thinking

[sensible]

Originally Posted by seenthelight!

Fwiw, 275x30x20 on the rears are an EXACT match for the 235x35x20 on the front ... No difference in diameter at all.

Yes these tyre sizes have the exact same rolling circumference. The tyre sizes suggested by BMW do not have the exact same rolling circumference but are within 1% (0.8%) so it could be argued that these tyre sizes are more suitable than those put on BMW''s new cars.

[Kermit]

Originally Posted by Skyhigh

Read this thread through: https://f30.bimmerpost.com/forums/sh....php?t=1688975

I must have missed something. I've read this all the way through and all it says is:

a) Poor chap is having difficulty with some leaks from and around the transfer box with a remap that's running close to the gearbox torque limits (fingers crossed you get it fixed soon demc1982); and
b) BMW Genius have confirmed that the tolerances in tyres across axles are stated for "control quality of suspension control systems and wheel alignment requirement". So basically wheel geometry. Nothing apparently to do with the transfer box, X-drive etc.

What's the connection? Where has this mythical issue with transfer boxes come from because someone hasn't told BMW yet.

[Skyhigh]

The connection is in the part which you have skipped, apparently - that it turned out there is a second leakage from the transfer case.

But the bottom line is that the information in that thread is tightly related to what is being discussed here, irrespective of why the the other thread originated on first place.

[Kermit]

No definitely didn't skip it if you read: "leaks" and "from AND around"... plural.

And no again, you are conflating issues by suggesting there's a link between tyres and transfer case issues, which there isn't any evidence for...?

[APJ]

Originally Posted by Kermit

No definitely didn't skip it if you read: "leaks" and "from AND around"... plural.

And no again, you are conflating issues by suggesting there's a link between tyres and transfer case issues, which there isn't any evidence for...?

Its okay mate, for some reason someone wants to make it more serious than it is or they have just woken up in a bad mood !, thought i was only asking a simple question. I personally don't have any clunks or bangs from the transmission, but because of all the information the nice chaps on here give, i want the car right. When i bought the car at two years old, the car came with those particular wheels, so that's what i am working with. And again because of the nice chaps on here, they have made my mind up to stick with the 275's that are on the car, be it that i will now have to change them to Michelins to be the same as the front, but i don't have a problem with that, just want the car spot on. Thanks guys

[HighlandPete]

Originally Posted by sensible

Yes these tyre sizes have the exact same rolling circumference. The tyre sizes suggested by BMW do not have the exact same rolling circumference but are within 1% (0.8%) so it could be argued that these tyre sizes are more suitable than those put on BMW''s new cars.

Just keep in mind that tyres have tolerances, BMW's star marked tyres are not necessarily to a nominal dimension, no more than Mercedes MO marked tyres are to the nominal generic size. Mercedes have stated MO dimensions can be different, in their technical docs.

When we say “exact”, we are talking ‘nominal’ dimensions for generic sizes. As “johnung” highlights, that is not meaning all tyres of a given dimension are to the nominal sizes. That is the issue with non approved tyres. BMW claim a high standard of 'uniformity' on Star marked tyres.

The RC typically given for generic sizes is to DIN 70020, which has a tolerance on the nominal RC figure of +1.5/-2.5% @ 37mph, (60km/h).

If you check the dimensions for some star marked tyre sizes, they are not running the nominal DIN Ø or RC.

[TiredGeek]

To put your mind at rest (or make you very unhappy, depending on the outcome) find a dead straight road/car park.
Mark the tyre wall with chalk at dead on 6 o'clock front and rear, drive a hundred yards, check the marks.
Hopefully they'll be in the same position front relative to rear, if one is pointing at 12 and the other at 8 then there's a problem with the rolling radius.
A small deviation you may be able to correct with adjusting the tyre pressure...…
Good luck