Car shifting funny after new wheels

[mastatikle]

I just put on my new 21" wheel set up and now under normal driving my car is shifting funny here and there. Like sometimes it holds revs higher or takes a while to shift up. "Manual" mode is fine. There is only a delay in auto mode, like it'll shift normal till 6th gear then won't go into 7th on highway by itself or take forever to do it under low efficient acceleration. Same issue going from 4th to fifth sometimes. Basically instead of shifting at 1700-2k it'll go all the way to 3k. Was I supposed to change tire size in computer somehow? Like OBDII.

[kong24680]

Originally Posted by mastatikle

I just put on my new 21" wheel set up and now under normal driving my car is shifting funny here and there. Like sometimes it holds revs higher or takes a while to shift up. "Manual" mode is fine. There is only a delay in auto mode, like it'll shift normal till 6th gear then won't go into 7th on highway by itself or take forever to do it under low efficient acceleration. Same issue going from 4th to fifth sometimes. Basically instead of shifting at 1700-2k it'll go all the way to 3k. Was I supposed to change tire size in computer somehow? Like OBDII.

I think it's just a coincidence that this happened after you switched wheels. Wheels should have no effect whatsoever with transmission shifts.

[mastatikle]

I don't know. It'd be an awfully close coincidence because it did it immediately after new wheel set up.

[mastatikle]

I almost want to put stock set back on to verify

[BMWDriverX]

Maybe try disconnecting the battery ground overnight or reset the ECU via the ODBII so the car goes back into adaptive learning mode. Might take care of the shifting issue.

[mastatikle]

Ok I'll try the battery dissconnect thing.

[upstate650]

If you maintained the same tire diameter the car has no idea what size your wheels are. Really, it doesn't have any idea what size your wheels are even if you did change the diameter. The only way wheels could have an effect is if you substantially increased the weight of the wheels. Substantially changing the width of the tires can effect handling characteristics at low/parking lot speeds but not transmission operation.

I'm with the coincidence crowd, and the hit the reset button crowd.

[mastatikle]

Originally Posted by upstate650

If you maintained the same tire diameter the car has no idea what size your wheels are. Really, it doesn't have any idea what size your wheels are even if you did change the diameter. The only way wheels could have an effect is if you substantially increased the weight of the wheels. Substantially changing the width of the tires can effect handling characteristics at low/parking lot speeds but not transmission operation.

I'm with the coincidence crowd, and the hit the reset button crowd.

Hmm I guess I'll switch them back just to make sure. And see how reset works. Should I just dissconnect negative side for a couple hours?

[upstate650]

Originally Posted by mastatikle

Hmm I guess I'll switch them back just to make sure. And see how reset works. Should I just dissconnect negative side for a couple hours?

that should do it. If it doesn't, maybe run it by the dealer and see if they will re-flash the transmission.

[lsturbointeg]

agree with what everyone else post. i think it's just pure coincidence. not sure how wheels could do that? try swapping your stock back on and see if it still does it

[mastatikle]

Well I went to BMW spoke with my SA. Said it could be an issue if the front and rear diameters are different. I'm going to put it on lift tonight and put stock ones on. There is right under a 2" difference right now

[brad850csi]

What tyre size did you put on?

[upstate650]

Originally Posted by mastatikle

Well I went to BMW spoke with my SA. Said it could be an issue if the front and rear diameters are different. I'm going to put it on lift tonight and put stock ones on. There is right under a 2" difference right now

yeah - what tires did you put on? how did you end up with a 2" difference in diameter??? Id like to see pictures of that.

[mastatikle]

Originally Posted by upstate650

yeah - what tires did you put on? how did you end up with a 2" difference in diameter??? Id like to see pictures of that.

Rear tire size is now 325/30/21 it looks great, and fills wheel well perfectly. Wheel manufacturer reccomend 325/25/21 but I wanted more sidewall for a better ride. I did however go with their recommended Front size of 255/30/21 not thinking at the time I should've upped that one as well. It's not too noticeable, still looks alright but I will definitely be going to 285/30/21 soon. I just hate to waste more money on this setup. I just had to buy new brake pads because of the insane amount of brake dust that shows on them. I can't figure out how to resize pictures using this website on my phone.. Lol

[brad850csi]

You need 285/35r21 or 275/35r21 to be close in radius. 285/30r21 is still quite small with that large a rear tyre.

[M6-Coupe]

Rear wheel with 325/30/21 tire is about 1.6" or 6% taller than OEM...
You would expect any unusual behavior from suspension/differential or transmission... I'm wondering how the handling is affected by this change!

[Evil Derboy]

If you up the wheels you need to reduce the tyres. The rolling radius on front and back should be the same. No wonder the cars acting weird. By having a difference between front and back surely the differential will be affected?

[mastatikle]

Originally Posted by brad850csi

You need 285/35r21 or 275/35r21 to be close in radius. 285/30r21 is still quite small with that large a rear tyre.

Damn, I hope 35 isn't too much sidewall. I might order the 285/30r21 first just to see if it fixes things. I really don't want more sidewall in front than the rear.

[mastatikle]

Originally Posted by Derboy

If you up the wheels you need to reduce the tyres. The rolling radius on front and back should be the same. No wonder the cars acting weird. By having a difference between front and back surely the differential will be affected?

No it's a M6 so it's only rear wheel drive. Meaning there's only one differential. Nothing ties the rear wheels to the front wheels other than the computer.

[mastatikle]

Originally Posted by M6-Coupe

Rear wheel with 325/30/21 tire is about 1.6" or 6% taller than OEM...
You would expect any unusual behavior from suspension/differential or transmission... I'm wondering how the handling is affected by this change!

Handling from the performance aspect I have not been able to really test because it's been ~60 degrees average where I'm at. But straight line, the car gets much more traction. I used to spin to 60 (top of 2nd gear) with 285/30/20 pilot supersports. Now I can grab traction in first!!! That's without an alignment and this cooler weather. Ride is SIGNIFICANTLY better. It was the first thing I noticed. Rides much much smoother, even with the smaller front tire. Turn in is a little better, but can't really prove that. I'm going up to 285 in front so not sure how turn in will be. Stopping should be improved though.

[brad850csi]

275*.35 = 96.25mm
285*.35 = 99.75mm

325*.30 = 97.50mm

[Evil Derboy]

Originally Posted by mastatikle

No it's a M6 so it's only rear wheel drive. Meaning there's only one differential. Nothing ties the rear wheels to the front wheels other than the computer.

That’s what I mean. If the computer is monitoring inputs from the ABS system which may also take into account data from the rear differential and given that the DCT system is computer controlled it’s stands to reason that given everything worked fine before, that your problem is due to you upsetting the balance of the car by changing the rolling radius between the two axles.

I’m not technically proficient enough to understand how or why but logic points to the wheel change being the problem. Interested to know what happens when you change them back.