Changing OEM tire diameter

[HighlandPete]

Originally Posted by snowbimmer

Great points, guys. It seems there would be some favorable tolerances built into the BMW 4WD systems, because there are thousands of folks here on the forums changing their rides from OEM tires and wheels to aftermarket products. (Granted, 2WD much less critical, so G8x M3s and M4s are newer to the game). I only seem to read about people with rubbing problems and strut clearances, which they fix, but never any differential problems (unless they just keep quiet).

Latest xDrive systems have been greatly revised, compared to the original designs, back in the days of models like the E83 X3. On those models tire tolerances were very critical and many transfer cases were compromised/destroyed with generic tire fitments. (Personal experience, my local tire shop would only fit star marked tires to the E83 X3, due to too many drivetrain issues with non approved fitments).

Transfer cases in the current M models have much more 'RWD only' engagement. The BMW M VTG control only applies torque to the front axle/wheels in limited situations. This means the multidisc clutch is running open much more of the time. This at least partly explains how BMW can be more flexible with M model tire selection.

Even in the more regular models, there are technical features to improve heat management and reduce wear. Plus changes to oil levels and flows, which reduce churning losses, aids efficiency.

[HighlandPete]

Originally Posted by shawnhayes

If you put them on the car, and mark where the tires are, and do 3-10 revolutions, if they are within 1% of the rotation (3.6 degrees) I feel pretty confident they'd be perfectly safe. You'd be shocked how much different 3.6 degrees x 10 rotations looks like. (Imagine 36 degrees of difference between the marks on the tires).

So, 1%, probably okay.

BMW have used the 1% in the past. They are very vague these days in spelling out specific technical tolerances. They hang more on recommending their approved tires. Plus the details of tire exchange/rotation and tread depth tolerances.

As you say, the rolling difference can be quite different. The difference has to be absorbed somewhere. Either in transfer clutch slip (if not wide open) and/or tire scrub.

[BMWCCA1]

Originally Posted by snowbimmer

As far as snow tires, sometimes you just need some effing snow tires

My comment was really that if you can afford what you obviously do drive, you can probably get something to drive in the snow, as infrequently as you have snow. I don't think a $150,000 8-series is you best choice. And if you don't like the ride quality, tires and tire sizes may get you where you want to go, but so will a Lexus where handling performance is decidedly secondary to comfort. I just hate seeing anyone compromise such a nice car and my day job is making sure my customers pick the right model of BMW for their purposes.

I have over 45-years experience doing this and, believe me, just last week I talked someone out of trading their X5 45e on an X5M simply because they want a little more power. Now realize they have an E39 M5 in the garage they never drive because "the value is going up so fast". And I can't count the number of people who insist they want an X4 because it's "bigger than an X3". Or those who think an X3M is a good idea because they want something more expensive than an X3 M40i. Talk about poor ride quality! Just sold a one-year-old X3 M40 on 20-inch wheels because the original owner hated the ride—but had to have those wheels.

That being said my E82 is on Plus-One Apex tires and DWS 06+ tires, because the Contis are good-enough in the dry and passable in bad weather (and I grew-up on lowered 2002s with Bilstein Sports!). But then the E82 also cost less than one-tenth of your car—and I can always drive my wife's E90 xDrive car with her Plus-One a/s tires if it snows that much.

You should love your car, even if you have to go with bigger donuts to get there.

[snowbimmer]

Originally Posted by BMWCCA1

My comment was really that if you can afford what you obviously do drive, you can probably get something to drive in the snow, as infrequently as you have snow. I don't think a $150,000 8-series is you best choice. And if you don't like the ride quality, tires and tire sizes may get you where you want to go, but so will a Lexus where handling performance is decidedly secondary to comfort. I just hate seeing anyone compromise such a nice car and my day job is making sure my customers pick the right model of BMW for their purposes.

I have over 45-years experience doing this and, believe me, just last week I talked someone out of trading their X5 45e on an X5M simply because they want a little more power. Now realize they have an E39 M5 in the garage they never drive because "the value is going up so fast". And I can't count the number of people who insist they want an X4 because it's "bigger than an X3". Or those who think an X3M is a good idea because they want something more expensive than an X3 M40i. Talk about poor ride quality! Just sold a one-year-old X3 M40 on 20-inch wheels because the original owner hated the ride—but had to have those wheels.

That being said my E82 is on Plus-One Apex tires and DWS 06+ tires, because the Contis are good-enough in the dry and passable in bad weather (and I grew-up on lowered 2002s with Bilstein Sports!). But then the E82 also cost less than one-tenth of your car—and I can always drive my wife's E90 xDrive car with her Plus-One a/s tires if it snows that much.
You should love your car, even if you have to go with bigger donuts to get there.

Well, it's not a 150k 8 series, it's a lowly M850. Still a lot of money, tho. And you say, while if I could afford this car I can probably get something to drive in the snow (true), it may not be true for the bloke that blew his wad just to get this car in the first place. Or, in my case, have room for the third car in the garage when you only have a 2 car garage and will not ever be parking a car that is not at least secured behind a locked garage door along with all my classified documents. My wife has an X3 (before that Jeeps and an MDX) and I will always have a sport sedan of some kind. It doesn't matter if you think it infrequently snows here, my wife and I are both away from the house a lot at the same time. While it may not snow a lot in my city, it snows a lot at my house - 1,000 above the city - and the difference can be no snow down below and several inches sitting in my driveway. I kinda need to get home if it starts snowing while I'm out. A lot of my neighbors park on a lower street and then walk up several hundred vertical feet to get home. I just drive home.

My 2006 530xi 6MT sent me down this road of being able to mostly have it all in a car: speed , luxury, performance, styling and kick ass traction. I don't baby my cars. If they get messed up, I fix them. If they wear out, I replace them. They have gotten roof boxes, bike racks, ski racks and many trips to the mountains for skiing, hiking and camping. My April trip to AZ last year should have been a breeze leaving Oregon. A freak snowstorm dumped several inches all over the state and I needed snow tires just for the first 2 days. I was driving at 8,000 feet down 395 in California at 28 degrees in a blizzard. The next day I driving through Death Valley. Not taking the X3 on a long trip. Ain't gonna happen.

My CA is like you - always selling a nearly new M something because someone had to have it and then didn't like the ride. You certainly have a really tough job when you have to deal with uninformed or inexperienced buyers. Especially when they think they really know what they want. When the X3 M40 came out at the end of our X3 lease, I test drove one with 21" wheels. Good grief, what a harsh ride. Tried one with 20" wheels - better. Tried the 19 variety and could live with it. But we bought her car off lease instead. Why mess with a known quantity if you like it. Still have it.

My original post was to inquire about the advisability of increasing an OEM tire diameter. A cushier ride due to a higher sidewall might be a secondary factor, but not a very important one. I ran summer Pirellis on my 19" snow wheels and summer Pirellis on my 20" OEM wheels with my M5. I thought I might be able to tell the difference in the ride, but I really couldn't. Both wheel sets went on several thousand mile trips.

My original statement still holds: The M5 and the M8 have the same tires and wheels. The M850 is the same big ass car as the M8 and is bigger and heavier and longer than the M5. It can go just as fast as an M5 on the freeway, just not as fast on a mountain road due to the suspension differences. I just think there should be an option where it can run with the same wider and taller tire set up of it's M brethren. But I guess that would infringe on the true M models (I think the other differences are vastly more important). Well, just get the M8, they say. My heart wants to be there, but this M850 really ticks more boxes for me. I never tire of getting in it, even for a 2 mile run to the store.