Changing OEM tire diameter

[snowbimmer]

Hey all, throwing this out there. M550s and M850s run a tire that is around 26.75" OEM diameter. The M5s and M8s run tires that are about 27.75" in diameter.

I've had the M550, two M5s and now have an M850. I feel the tires are just to small for this car - aside from being smaller by an inch in diameter, they also run 8" fronts and 9" rears - as opposed to the M5 & M8 at 9.5" fronts and 10.5" rears.

So, in the spring I am looking at new Vossen wheels and going to 9" fronts and 10.5" rears. I'd like to go up one profile, too, to gain that other inch in height (the wheel wells really need to be filled up a bit more - and it would ride a teensy bit better with more rubber in the side wall).

I took my M5 snow set down to the tire shop to have a look with them on the M850. They mostly fit, however the offsets are just a bit off, but the inch larger tire really looks good. They said a different wheel with the taller tire would be no problem.

I went to my BMW dealership and talked to the tire guy. He waffled about going up a tire size. He checked with the service techs. They said no, don't mess with the OEM diameter. I went to my performance shop guy where I am getting the new wheels. He said don't mess with the OEM diameter. With all the computers, AWD, DSC and what have you, the car could possibly not perform correctly.

If you keep the front and rear wheels within a 1% diameter tolerance between axels and go up a profile size, does the car really care if everything is rolling along, but at a 3% or so, different rate of speed on the axel rpm? I know people have gone wider all the time, but you never really hear about changing diameters that much. I know people do this, just don't know what the consequences are, if any, aside from a speedometer that will be slightly off. (I think it would bring the speedo speed closer to the true speed of the car, thus offsetting the 2-3 mph difference built into BMWs - if I remember which way the math works for this application. I could be wrong - maybe it's the other way around, but I don't feel like checking it now.)

Any thoughts appreciated.

Thx

[Atavus]

BMW or any other German company will expect you to stick to the wheels and tyre sizes they have in their approved list.

If you keep the front and rear circumference as close as the standard tyres are, the car can’t be any the wiser.

Will it give you 100% of the experience BMW designed, who knows?

[Llarry]

Originally Posted by snowbimmer

Any thoughts appreciated.

I see where you are coming from here and count me among the few who thinks you could be on to something. When considering stagger, I always like to work with revs per mile for a particular tire rather than diamrter; revs/mile is a bit more precise. (I use mfr specs or Tire Rack to get revs/mi.)

So, for instance, going up one size in front and rear (and comparing revs per mile to make sure you are not going to mess up your electronic nannies) will almost always work, at least in the electronic sense. Clearance in the wheel well is a separate issue but generally one size up won't cause problems; working with a good tire shop helps here for test-fitting or whatever.

I'm not conversant with the 8er but offhand I see two OE options: 19s with 245/40R-19 & 275/35R-19 and 20s with 245/35R-20 & 275/30R-20. Oh, and now I see you you're proposing a 1.5" width bump both front and rear and an inch wider tire (?) -- that's 25mm -- now you've lost me; I think that is just too extreme.

I would think going to 255 front and 285 rear would not be imprudent, but what you're talking about is pretty radical. My two cents.

[335i54n]

just keep rolling diameter same front and rear close and you be fine. more front tire always helps imo

[snowbimmer]

Originally Posted by Llarry

I'm not conversant with the 8er but offhand I see two OE options: 19s with 245/40R-19 & 275/35R-19 and 20s with 245/35R-20 & 275/30R-20. Oh, and now I see you you're proposing a 1.5" width bump both front and rear and an inch wider tire (?) — that's 25mm — now you've lost me; I think that is just too extreme.

I would think going to 255 front and 285 rear would not be imprudent, but what you're talking about is pretty radical. My two cents.

Yes, I'm going 255 front and 285 rear.......only 10mm wider per tire. But the front rim will gain an inch from 8" to 9". The rear rim will go from 9" to 10.5". They are 20" dia wheels.

The M8 runs a 275 on a 9.5" wheel in front and a 285 on a 10.5" wheel in back. The width isn't the problem. My question still remains about the new height vs OEM.

[snowbimmer]

Originally Posted by 335i54n

just keep rolling diameter same front and rear close and you be fine. more front tire always helps imo

That's what I am thinking. By "more front tire" you mean width, correct?

[Llarry]

Originally Posted by snowbimmer

Yes, I'm going 255 front and 285 rear.......only 10mm wider per tire. But the front rim will gain an inch from 8" to 9". The rear rim will go from 9" to 10.5". They are 20" dia wheels.

The M8 runs a 275 on a 9.5" wheel in front and a 285 on a 10.5" wheel in back. The width isn't the problem. My question still remains about the new height vs OEM.

Well, the new "height" (or revs per mile) will change the effective final drive ratio of the car. Your speedometer and odometer will no longer read the same as they did from the factory. Note, however, that German speedometers are deliberately calibrated to read slightly high. Your speedometer may well be slightly more accurate with the larger tires. (Edit: The change will, however, be tiny.)

I don't see a problem with your intended course of action, if -- and only if -- the wider rims don't rub on suspension components, fenders or wheel well. That's where the part about a cooperative and knowledgeable tire seller/shop comes in.

[snowbimmer]

Originally Posted by Llarry

I don't see a problem with your intended course of action, if — and only if — the wider rims don't rub on suspension components, fenders or wheel well. That's where the part about a cooperative and knowledgeable tire seller/shop comes in.

I fooled around a lot on "Will it fit?" once I decided on a wheel and knew the offsets. Then took my ideas to my performance shop. He calls his wheel guy, tells him I have an M850 and he tells my guy the wheels and tire size I should use.......they have already test fitted their various wheels to see which works for each make and model. It took him about a minute and a half. We were bang on together, except it took me hours of screwing around. But, he's recommending I stay at OEM height.

The wheels end up about as if I was running a 10mm spacer on my OEMs. Nothing real radical. I just want more meat at each corner and then a little taller if it's doable.

Thx for your comments.

[BMWCCA1]

Increasing overall tire height (diameter) with the same wheel diameter will only decrease the handling capability of your car with respect to turn-in due to slip-angle. If you're looking for increased comfort, you shouldn't have bought a sport model in the first place! Just get a 530i xDrive and leave those snow tires off your M5!

[Llarry]

Originally Posted by BMWCCA1

Increasing overall tire height (diameter) with the same wheel diameter will only decrease the handling capability of your car with respect to turn-in due to slip-angle. If you're looking for increased comfort, you shouldn't have bought a sport model in the first place! Just get a 530i xDrive and leave those snow tires off your M5!

I think the OP is more concerned with appearance.

[DanG]

The M2 crowd has changed the wheel and tire size. As an example I went from 19X9 and 19X10 to 19X9.5 and 10X10.5 BBS wheels. I increased the tire size from 245/35/19 and 265/35/19 to 255/35/19 and 275/35/19.

BBS called my wheels “Tuner Fitment”. I am not running any spacers and I have zero rubbing.

[snowbimmer]

Originally Posted by BMWCCA1

Increasing overall tire height (diameter) with the same wheel diameter will only decrease the handling capability of your car with respect to turn-in due to slip-angle. If you're looking for increased comfort, you shouldn't have bought a sport model in the first place! Just get a 530i xDrive and leave those snow tires off your M5!

I have been on the elusive search for the "perfect" car for several years. I want fast, I want luxury, I want canyon carving handling and I want drivability in the city. The M550 was great, but the M5 beckoned. My base 2019 M5 proved to be too stiff in the city. My 2021 LCI Comp was spectacular, fixing many of the problems with the base - but, still too jarring (for a 130k car) in the city for me. The M850 is still stooopid fast, luxurious, rides better - at the expense of giving up some of the suspension goodies. It's also a bigger car and a heavier car. I don't know why they didn't give it the same tires and wheels as the M8. The OEM tires are really low profile. I don't know why you think I shouldn't have bought a sport model, when all I'm trying to do is get back to the size tires that were on my M5, and are also on the M8.

I've taken many 4,000 mile two week trips in the last few years. We go 300 miles/day and try to stay off the freeways. The car is a blast on the two laners, but obviously not M5 territory. It's most noticeable when I push it, but that's not really all that practical with someone else in the car telling you to slow down. (Sheesh. Women...) The wider wheel I am also going to would help negate any increase in tire height.

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


By the way, these snow tires were an 8" square set from my M550 - and they sucked for a car this big. But, they got me there and back. I bought the car car 2 days before the trip and it's all I could do. Luckily they were still in my basement. But, they are also the recommended BMW snow set for the M850.

Same trip, the next day


This winter I am running Michelin Alpin4 tires on the OEM wheels.........much, much, much better.

Originally Posted by Llarry

I think the OP is more concerned with appearance.

Its a twofer: more sidewall plus less wheel gap. I doubt is there is a single M8 owner that says, "Ya know, Ima gonna go down a profile. I just want more space between my fender and the tire."

[snowbimmer]

Originally Posted by Onesie

The problem with your car is the xdrive.
You can change the tire diameter but you need to keep all 4 in the same ratio as stock.
The M8 uses different diameters front/rear but it’s setup like that. That’s why you can’t go as wide in the same proportion as the M8.

Figure out the original proportions, up 1 size on both axles and you should be good to go. Mind the fact the car will drive a bit differently and not necessarily better. I’ve done this before (not in an 8) and I prefer the stock setup, but YMMV.

I think I see where you're coming from. I've investigated this a lot. The M5 and M8 run with rears slightly bigger in diameter
Front Dia: 27.58" Rear Dia: 27.86"

The BMW M5 & M8 snow set uses a 19" wheel and goes down a width to 265 in front, making the gap wider
Front Dia: 27.34" Rear Dia: 27.98"

The 19" OEM summer set up (which I also ran) was
Front Dia: 27.66" Rear Dia: 27.98"

In all cases the rears are between 1% to 2% larger in diameter. So, my question, since you bring up x-drive, does BMW engineer the car
for this larger wheel diameter in the rear - OR, do they just find the tires that fit within certain tolerances and call it a day?


The M850, on the other hand is just the opposite: The fronts are a larger diameter than the rears
Front Dia: 26.76" Rear Dia: 26.50"

My new setup with 10mm wider tires, and keeping as close to OEM sizing as possible take me to:
Front Dia: 27.03" Rear Dia: 26.73"

I could then go up one profile, but then I'm pushing 28" diameters, but I could still keep the fronts slightly taller than the rears.

And, again, did BMW engineer the M850 with the expectation that the front tires are to be taller than the rears? The Alpina B8, which is a souped up M850,
actually uses the taller tires. like the M5 and M8.

I think I'll probably just stay within OEM sizing and call it good.

[BeEmVe]

If you have not used the tire calculator before here is a link: https://tiresize.com/comparison/

[snowbimmer]

Originally Posted by BeEmVe

If you have not used the tire calculator before here is a link: https://tiresize.com/comparison/

Thanks. I have several I use.

This is also good for quick and dirty
https://www.discounttire.com/learn/tire-size-calculator

This is good for offsets and poke calcs
https://willtheyfit.com/

[HighlandPete]

Originally Posted by snowbimmer

The M850, on the other hand is just the opposite: The fronts are a larger diameter than the rears
Front Dia: 26.76" Rear Dia: 26.50"

....

And, again, did BMW engineer the M850 with the expectation that the front tires are to be taller than the rears?

Something to keep in mind. Are you calculating the diameter from generic sized tire charts/calculators? That is not 'definitive' sizing for 'star' marked approved tires.

Also remember those are nominal sizes, not allowing for manufacturing tolerances, (even within the appropriate standards), can be quite wide. BMW could well be setting tolerances which may make front and rear RC (rolling circumference) for the OE tires different than it appears.

[shawnhayes]

We had to learn this the hard way in the GT-R world. Many manufacturers and tire size calculators that were "just right" still blew center differentials over and over.

A guy named Shep from ShepTrans (famous in the EVO/DSM/GT-R world for drivetrain) finally gave us the reality. Only way to know for sure is to install the damn tires, and put them on the car at running pressure, and mark them and do a slow rollout.

Unless they were REAL close, you'd eventually blow the center diff.

BMW's system is probably a little more tolerant. Who knows? I don't be the one to find out. I install either BMW "listed" tires and setups, or I install "square" (same tire all the way round) setups.

OP, reality is you can change the OD, but lots of unpredictables can occur as the result. Just giving you a suggested pathway to ensure safety.

Shawn

[snowbimmer]

Originally Posted by HighlandPete

Something to keep in mind. Are you calculating the diameter from generic sized tire charts/calculators? That is not 'definitive' sizing for 'star' marked approved tires.

Also remember those are nominal sizes, not allowing for manufacturing tolerances, (even within the appropriate standards), can be quite wide. BMW could well be setting tolerances which may make front and rear RC (rolling circumference) for the OE tires different than it appears.

Originally Posted by shawnhayes

We had to learn this the hard way in the GT-R world. Many manufacturers and tire size calculators that were "just right" still blew center differentials over and over.

A guy named Shep from ShepTrans (famous in the EVO/DSM/GT-R world for drivetrain) finally gave us the reality. Only way to know for sure is to install the damn tires, and put them on the car at running pressure, and mark them and do a slow rollout.

Unless they were REAL close, you'd eventually blow the center diff.

BMW's system is probably a little more tolerant. Who knows? I don't be the one to find out. I install either BMW "listed" tires and setups, or I install "square" (same tire all the way round) setups.

OP, reality is you can change the OD, but lots of unpredictables can occur as the result. Just giving you a suggested pathway to ensure safety.

Shawn

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)

I can only base my most experience off my 2 M5s. The OEM 20" summer wheels have a front to back diameter difference of 0.28", or 1.02%.

The OEM 19" summer set has a front to back diameter difference of 0.32", or 1.16%.

The BMW approved 19" snow set has a front to back diameter difference of 0.64", or 2.34%.

(The above based on nominal sizing, because I have no other data to go on)

"They" say to always have your front to back diameters within 1% on an AWD car. The OEM summer 20s and 19s are close to 1% (still over), but the winters are are considerably over a 2% difference in diameter. I saw no ill effects during 3 winter seasons, so I'm thinking we may have a little leeway here.

And shawnhayes, what did you mean by "REAL" close when you do the roll out? How close is REAL close in Shep's world to not blow out the center diff?

Thanks for the input. This is what I've been looking for.

[Accentu8]

I have been driving on B roads and freeways as well as taking some quick corners for 5 days now and I’ve noticed no detriment to handling and ride is much better and much quieter with 40 front and 35 rear sidewalls. I don’t have xdrive. I love the new Continental ExtremeContact DWS06+. It’s an ultra high performance A/S that Tire Rack rated very high and it has great customer reviews as well. I’m very happy with my choice and filled up the wheel well nicely as well as increasing ride height by 1/2” which in these low slung cars certainly not a bad thing.

[snowbimmer]

Originally Posted by 23TripleBlack

I have been driving on B roads and freeways as well as taking some quick corners for 5 days now and I’ve noticed no detriment to handling and ride is much better and much quieter with 40 front and 35 rear sidewalls. I don’t have xdrive. I love the new Continental ExtremeContact DWS06+. It’s an ultra high performance A/S that Tire Rack rated very high and it has great customer reviews as well. I’m very happy with my choice and filled up the wheel well nicely as well as increasing ride height by 1/2” which in these low slung cars certainly not a bad thing.

Ah, no xDrive. I doubt you'll have problems, then.

Damn, that sure looks good. I had a Donington M5 - my favorite color. Skyscraper is close.

[shawnhayes]

Originally Posted by snowbimmer

And shawnhayes, what did you mean by "REAL" close when you do the roll out? How close is REAL close in Shep's world to not blow out the center diff?

Thanks for the input. This is what I've been looking for.

For the GT-R (way different system than the x-drive, but I don't understand all the nuances yet, but the suggestion is that the weak point is the center transfer case - not a heavy duty piece frankly), the 1% was certainly within what Shep was talking about. But, that's PRESUMING what you're TOLD is actually TRUE. Manufacturers LISTED measurements are frequently "meh" from what we found.

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.

Shawn

[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.