[jlc45]

Just curious. I see posts where folks are using different size tires on front and rear, and that might be Ok, but is it not required to have same overall diameter on all tires on a car with X drive? E.g. 245 x 40 x 19" tires will have 678 mm diameter, 26.7". It's OK to use a slightly different size, but shouldn't all four have same diameter (as each other). Thanks.

[Surly73]

Originally Posted by jlc45

Just curious. I see posts where folks are using different size tires on front and rear, and that might be Ok, but is it not required to have same overall diameter on all tires on a car with X drive? E.g. 245 x 40 x 19" tires will have 678 mm diameter, 26.7". It's OK to use a slightly different size, but shouldn't all four have same diameter (as each other). Thanks.

Very important. It's a clutch type transfer case and it will burn if there is too much difference. I don't know that BMW has ever published an official spec, but I think I have heard 1% rolling circumference.

It's worth also noting that in cases where staggered configurations are available from the factory, I bet you'll find that those tires are available in those sizes with "star designations" indicating "BMW OEM". BMW likely ensured that those sizes with the stars end up with very close rolling circumference when used in staggered configuration with X-Drive. If you don't get the star version this may not be the case.

[HighlandPete]

Originally Posted by Surly73

It's worth also noting that in cases where staggered configurations are available from the factory, I bet you'll find that those tires are available in those sizes with "star designations" indicating "BMW OEM". BMW likely ensured that those sizes with the stars end up with very close rolling circumference when used in staggered configuration with X-Drive. If you don't get the star version this may not be the case.

Interesting you say the above. The E83 X3 was particularly susceptible to drivetrain issues. With star marked approved tires, no real issue. Same size generic tires could cause transfer box problems and drivetrain jerking.

My son ran a couple of E83 with staggered setups. When he needed new rear tires, for his 19" wheels, local supplier (a national outlet) didn't have approved tires. The manager wouldn't fit an alternative he had in stock, as he'd experienced too many issues doing so. He couldn't source any approved tires, (was a shortage nationwide at the time), but was willing to fit, if my son could source any.

Long story short, not all tires are equal in dimensions. 'Star' marked approved tires can have different tolerances. Same with Mercedes and MO approved tires.

[Surly73]

Originally Posted by HighlandPete

Long story short, not all tires are equal in dimensions. 'Star' marked approved tires can have different tolerances. Same with Mercedes and MO approved tires.

To be exceptionally clear, what I'm saying is that BMW will have ensured that for STAR marked tires, the popular staggered pairings will be close enough in rolling circumference as long as they are STAR rated.

So for example the F10 535 with a staggered set up uses 245/40-19 and 275/35-19. I bet if you look through tire catalogs you'll happen to notice that BMW has star-approved tires in those sizes and will have ensured that the two are within acceptable transfer case limits.

You might find that those two sizes (even same make/model) might NOT be close enough in rolling circumference for a BMW transfer case if they are not star-marked. All of the dimensions provided in tire sizing are approximations and ranges, just like with TVs, shoes, pants, grades of motor oil etc...

A 1992 Porsche I had a long time ago came staggered from the factory. Porsche also has an "N" marking for approved tires. Of course most of us owners thought that was a scam to earn profit or drive tire sales through dealers. We also thought that fitting a wider profile would be "better".

Long story short, instead of 205F and 225R I fitted 225F and 245R Bridgestone S-02 Pole Positions when I needed tires. The size specification was bang on according to tire calculators, and some folks noticed nothing odd, but it turns out that Bridgestone made the 245 tire on the "short and narrow" end of the tolerances and the 225 tire on the "wide and tall" end of the tolerances and it looked stupid instead of looking balanced. No transfer case to worry about at least in that case.

[jlc45]

So where does this star marking appear? (Interesting)
Is it actually on the tires themselves?
The two sizes Surly quoted above are theoretically 0.5% different in diameter (and circumference, amazingly), but I imagine a worn tire versus a brand new tire is at least that much different.

[HighlandPete]

Originally Posted by Surly73

To be exceptionally clear, what I'm saying is that BMW will have ensured that for STAR marked tires, the popular staggered pairings will be close enough in rolling circumference as long as they are STAR rated.

I'm on the same page. Perhaps I should have said "'Star' marked approved tires can have different and/or tighter tolerances".

The tolerance within DIN standard for RC is; +1.5% -2.5%. Apply that to tires BMW are using for staggered xDrive combinations, we can see BMW may well have much tighter (or different) tolerances for approved tires.

[HighlandPete]

Originally Posted by jlc45

So where does this star marking appear? (Interesting)
Is it actually on the tires themselves?
The two sizes Surly quoted above are theoretically 0.5% different in diameter (and circumference, amazingly), but I imagine a worn tire versus a brand new tire is at least that much different.

Nominal size doesn't mean that is the actual difference, could be far more at the extreme of the 'generic tire' tolerances. That's where the approval standard likely comes into play.

Regarding wear, BMW have indicated that a maximum 2mm of tread difference between wheels is the figure we should work within.

Something like this;

"The tire tread difference between tires in all wheel positions must not exceed 2mm".

[Nickco43]

When custom sizing tires I typically will not go above 1% circumference difference from the factory tire size.

I use this calculator: https://tiresize.com/converter/

[Surly73]

Originally Posted by Nickco43

When custom sizing tires I typically will not go above 1% circumference difference from the factory tire size.

I use this calculator: https://tiresize.com/converter/

That's a great guideline when going to something non-standard to keep your speedometer reasonably accurate, but isn't what is being spoken of. On an X-Drive you could be 5% off of the original spec as long as all four are the same and all that would suffer is your speedometer reading (and perhaps rubbing if you went larger).

[Surly73]

Originally Posted by HighlandPete

Regarding wear, BMW have indicated that a maximum 2mm of tread difference between wheels is the figure we should work within.

Something like this;

"The tire tread difference between tires in all wheel positions must not exceed 2mm".

Ah - so that allows us to work it out, and it's actually more like 0.5%.

They've said a difference of 2mm RADIUS is their limit, equal to 4mm of diameter.

Tiresizecalculator reports the nominal diameter of 245/40R19 is 678 for a cirumference of 2131. A reduction in 4mm of diameter would reduce circumference by ~12mm or 12/2131=0.6%

[HighlandPete]

Originally Posted by Surly73

Ah - so that allows us to work it out, and it's actually more like 0.5%.

They've said a difference of 2mm RADIUS is their limit, equal to 4mm of diameter.

Tiresizecalculator reports the nominal diameter of 245/40R19 is 678 for a cirumference of 2131. A reduction in 4mm of diameter would reduce circumference by ~12mm or 12/2131=0.6%

BMW state in their service data and for for xDrive Tire Tolerance Logic... Something like;

"The tire circumference can fluctuate up to 1% as a result of mixed tires or wear".

[jlc45]

That was smart of you to pick up on radius versus diameter. And as I'm sure you already know, it doesn't matter whether you're talking about diameter or circumference since the two are directly proportional. A 1% increase in diameter will be a 1% increase in circumference also. (Thanks for answering my original question. You confirmed that my expectation was correct; that all four tires (or tyres, for Pete) should be "the same" in diameter plus or minus a small tolerance. On an X drive.

[Hoa]

I have xDrive and have been running a staggered setup. So far no issues

Front 245 40 19
Rear 275 35 19

[cals]

yes, you can run staggered wheels in an X-Drive, providing the diameter difference between Front and Rear wheels stays below +/- 1%

The BMW Manual lists several sizes (on this case for a 535ix); I have a set in the 19" size.

[TEF]

On a 5 series xdrive sedan what are the advantages of staggered wheels/tires aside from appearance? It's understandable on a rwd sports cars, but I don't see where this would aid in the handling of a awd sedan.

[jlc45]

Hoa, you have tires where all four are almost exactly the same diameter overall. You have different widths and aspect ratios, but they are all the same diameter, within the allowable tolerances. The original question, which was answered, was about diameter and therefore also circumference. In an x drive you want all four wheels turning at the same rpm or you might mess up the transfer case.

[Unspec]

Originally Posted by cals

yes, you can run staggered wheels in an X-Drive, providing the diameter difference between Front and Rear wheels stays below +/- 1%

The BMW Manual lists several sizes (on this case for a 535ix); I have a set in the 19" size.

I'd be a little cautious trusting that manual though. 245/35/20 F and 275/30/20 R is -1.1%

245/35/20 F and 285/30/20 R is a lot safer at -0.4%

[cals]

that is a good catch !!!

just for curiosity, checked on tire rack for my car and indeed, the 20" shows the 285/30 size !!!

.

[Unspec]

Originally Posted by cals

that is a good catch !!!

just for curiosity, checked on tire rack for my car and indeed, the 20" shows the 285/30 size !!!

.

Likely a mess up on BMW's part, there's occasional mistakes in their manuals. Always double check!

[HighlandPete]

Originally Posted by Unspec

I'd be a little cautious trusting that manual though. 245/35/20 F and 275/30/20 R is -1.1%

245/35/20 F and 285/30/20 R is a lot safer at -0.4%

The 275/30 R20 is the correct rear size for a BMW staggered setup on F10 & G30 models. Remember we are talking approved tires, not necessarily generic nominal tolerances.

[Unspec]

Originally Posted by HighlandPete

The 275/30 R20 is the correct rear size for a BMW staggered setup on F10 & G30 models. Remember we are talking approved tires, not necessarily generic nominal tolerances.

Sure, if you only ever buy star tires. But for people who don't and don't know where to check besides the manual, that 275 is highly misleading.