[pmsteinm]

I just started looking in the E90 tech guide for the ATC300 transfer case. I think that is where the "1%" comes from:

"The tire circumference can fluctuate up to 1% or more as a result of mixed tires or wear. The tire tolerance logic decides depending on the driver's command and driving situation whether the slip is to occur in the transfer case clutch or at the contact area between tire and road.

If the slip is permitted in the transfer case clutch, the locking pressure set by the pre-control is reduced in order to keep the work loss low. In the driving dynamic control situation, the clutch is locked slightly more than normal, the four wheel drive is always guaranteed when required.

For maximum xDrive performance, tires (and wheels) of the same diameter should be installed on the vehicle."

[BMWbiker]

I have read in several sources that star tires for BMW, and the equivalent for several other manufacturers (e.g. M-B), are different from the standard spec tires for that model.

One may or may not care about the differences, but I gather that they do exist.

[chris3g]

Originally Posted by pbonsalb

I read somewhere that one of the star spec Michelins has 0.5mm less tread than the non star spec. I forget which size/application. I don’t believe that was for transfer case longevity; I think it was more likely for ever slightly better handling.

Somewhere there is a post that says BMW pulls the Michelins and installs Continentals on M5’s at their 2 USA driving schools. That was from a guy who did one of the schools.

For the M5, BMW allows square snows — all 265/40/19. So 0% difference is OK instead of 1%. BMW also allows 265/40/19 and 285/40/19 snows, so 2% is OK instead of the stock 1% difference. Seems like the 0-2% range is OK for the transfer case, ABS and traction control. I personally would be hesitant to go further in either direction.

My take on the above is that non star spec vs star spec tires in the same sizes won’t make any difference. I may put Continentals on my M5 next year. Not sure yet — it will depend more on size availability in the wider sizes I want than on star or no star.

I am also not scared of mixing similar level tires front to rear. I would not mix side to side. And I would not mix different level tires front to rear (i.e., cheap all seasons in front and PS 4 on the rear). But who would?

All the cars at the performance center are on Conti tires, because continental is currently the main sponsor of the performance center. they pay BMW to have their tires on all the cars there.

[pbonsalb]

Which means that non star spec tires won’t damage or adversely affect the handling of your car — BMW is using non star spec tires under the most adverse conditions.

[BMWCCA1]

I was surprised to see the first post in this thread comparing different brands rather than star to non-star tires from the same brand.

I can't say if it's still relevant today but years ago when the E34 was the current 5-series, a good customer came to me after replacing his original Michelin MXV-4s with the "same" tire without the star from the local Michelin dealer. He was complaining about the change in handling and so we compared the still-original BMW spare tire with the new store-bought Micheins. Right there on the sidewall showed the non-star tire was constructed with two-fewer plies in its carcass than the OE star version. Obviously there was a noticeable difference in sidewall flex which resulted in a larger slip angle. And these tires, other than the star, were the same spec, brand, and model.

[pbonsalb]

Star are different in some or maybe all cases. There should be no dispute about that. Might be just a change to improve the load rating or might be a tread compound or design difference. But that does not mean your BMW will perform poorly with non star tires.

I recommend doing some research on what others have used or what should work well. For the Michelin PS4 with star for the F90 M5, it appears there is a Continental tire that is a great alternative — even BMW is using it.

If you don’t know what you are doing or don’t want to research, buy star tires. No one is saying they are not a good choice. Some of us are simply saying there may be additional choices that may be considered.

[M5Rick]

On rear drive M cars it doesn't matter one iota if you have a star or not. I have Porsche designated summer 305 PS4s's on my rears and I'm happy.

[jmg]

Originally Posted by mjr24

This is ridiculous. The star tires are just their way of getting you to buy “special tires” that are ridiculously high priced and wear out fast.


It’s akin to when a dealer says only they should change your oil and no one else.

There is zero difference between star tires and normal non-star tires…..at least not one that you or I will ever notice.

This is incorrect. You merely have to feel the outer corners of the * spec vs the non-* spec. It has a different composition.

Originally Posted by user12464

FWIW BMW uses non star spec Continentals at all the M school cars driven on the track (including the X-Drive M5). They then go on to sell the school cars as CPO. So clearly even BMW doesn't think star spec is crucial.

BMW uses Continentals on M School days because they sponsor the M school. If you go to a non-M School, like teen car control, you will see Michelins. But you are right, they are not "crucial". They are, however, more ideal for what BMW was trying to accomplish out of the factory.

Originally Posted by M5Rick

On rear drive M cars it doesn't matter one iota if you have a star or not. I have Porsche designated summer 305 PS4s's on my rears and I'm happy.

It does not matter on AWD vehicles either. You just have to run the same tire diameters on all 4 corners.

Originally Posted by BMWbiker

Just to confuse things a bit more, the G26 M440i x-drive GC comes with:

19x8.5, 245/40 R19 front
19x9.0, 255/40 R19 rear

This is more than a 1% difference in diameter.

The difference is 1.18% or 0.315 inches. Some source say 2.5% is the max. I've heard 1%. Are there problems with this setup in the M440i?

[T.R.S.]

Originally Posted by BMWCCA1

I was surprised to see the first post in this thread comparing different brands rather than star to non-star tires from the same brand.

I can't say if it's still relevant today but years ago when the E34 was the current 5-series, a good customer came to me after replacing his original Michelin MXV-4s with the "same" tire without the star from the local Michelin dealer. He was complaining about the change in handling and so we compared the still-original BMW spare tire with the new store-bought Micheins. Right there on the sidewall showed the non-star tire was constructed with two-fewer plies in its carcass than the OE star version. Obviously there was a noticeable difference in sidewall flex which resulted in a larger slip angle. And these tires, other than the star, were the same spec, brand, and model.

I've never had a BMW before and just ordered a M240i X-drive a few weeks ago. On previous "sporty" vehicles I've owned when replacing tires I've always gone with XL (extra load) rated tires as felt they offered additional sidewall strength that resisted deflection more, and then also offered some additional protection to the lower profile tire and the rim it was mounted on from ass-puckering potholes that are not particularly uncommon in later winter into spring in my area due to highly the variable weather conditions (and then sometimes just pop up in summer for whatever reason).

[BMWbiker]

Originally Posted by jmg

The difference is 1.18% or 0.315 inches. Some source say 2.5% is the max. I've heard 1%. Are there problems with this setup in the M440i?

Not that I know of. It just seems a bit odd to me for BMW to emphasize the importance of having the tires be the same diameter, then selling the car with a tire combo that isn’t.

I’m still breaking mine in, so I’ve mostly been driving it like a stereotypical grandmother, and definitely staying under 4K RPM (per the owners manual).

[pbonsalb]

I assume there is some sort of electronic control that can be programmed to inform the transfer case of the front to rear difference. Does anyone know?

[locrianmod]

Novadays I'm researching alot also, I will choice oem tire when replace, * will guarantee to engineering and fine tunning end to end match. Also About run flat tech, it is really important to me for high speed safety!

[pmsteinm]

Originally Posted by pbonsalb

I assume there is some sort of electronic control that can be programmed to inform the transfer case of the front to rear difference. Does anyone know?

I don't think it is needed. There is no reduction gearing or anything so if the axle speeds are different the clutch needs to slip. There is some text in the guides that allude to it detecting that and setting the clutch a little less to allow the slippage to occur more smoothly. This would imply that if you have mismatched sets there is greater slippage so the AWD can be less effective at transferring power to the front.