[rothwem]

Sorry, this is a bit of a long-winded post, so here’s a TL: DR summary:
-Put oversized tires on
-Shifting sucked on auto trans
-Put stock size tires on
-Shifting is great
-Can tire size be coded into the DSC module?

When I bought my car, it had the sport package staggered wheels with basically worn out tires. I think that staggered wheels on a 230hp wagon with 50/50 weight distribution is silly, so I sold them and picked up a super cheap set of 16" wheels and tires off the list of Craig within my first week of ownership. The plan was that I would use up the cheap Mastercrafts that came on the 16s, then I'd put winters on and they would be my winter tire set.

The tires were 225/55/R16 instead of the stock size of 205/55/R16 since the wheels were off of an E60. I've used 245/45/17s on my old 6MT E91 (same rolling diameter) so I knew this size would fit. They looked cool and meaty, however, I underestimated just how awful craigslist Mastercrafts would be. Sloppy and floppy was the name of the game, with poor wet traction and poor dry traction as well. I lasted about a month before I took them off to put a set of Bridgestone RE980AS tires on that are approximately 1.7 million times better than the Mastercrafts.

The oddest thing though, was that the automatic shifting of the car improved an incredible amount. With the oversized tires, the car would bog then downshift at odd times, so I had resigned myself to just using the DS mode, where everything worked better, thinking that it was just the odd BMW trans tuning for MPGs, or maybe I needed to change the transmission fluid. With the new tires, it seems to be in the right gear all the time, and its actually fine to drive in "D" mode.

I googled, and apparently this is really common on lifted trucks, where guys put 33s on a truck that originally had 30.5s. Its apparently because the trans uses the speed signal to figure out when to shift, so it gets thrown out of whack when the oversized tires are put on the vehicle and the speed signal is wrong.

There's all kind of products to adjust the speed signal on F150s and Jeeps, but nothing for a BMW. It then occurred to me that there's probably some sort of way to code it to the vehicle, especially since the ECM and probably the DSC module are similar between the E60 and E90, and the E60 uses larger tires. I went through the DSC module TRC files to see if there was some sort of parameter I could alter, but I couldn’t figure it out.
Has anyone had any luck or experience with changing the rolling circumference in the vehicle’s electronics?

[juld0zer]

It would be less effort to fit the correct size tyres.
DCT M3 are known for refusing to downshift via manual (paddle or lever) or having very delayed downshifts if there is a large enough difference in rolling circumference across the axles. Even improper tyre pressures can cause it. Non M models are less sensitive

[rothwem]

Originally Posted by juld0zer

It would be less effort to fit the correct size tyres.
DCT M3 are known for refusing to downshift via manual (paddle or lever) or having very delayed downshifts if there is a large enough difference in rolling circumference across the axles. Even improper tyre pressures can cause it. Non M models are less sensitive

Well yeah. Obviously less effort, but where's the fun in that?

Besides, I'm kinda fixated on this idea of an Allroad style E91. I joked about it when I had an xDrive with new suspension that sat up really high, but now that I'm in the mountains and I drive on gravel roads several times a week, it doesn't seem that crazy.

[rjahl]

Don't know too much about the the GM gearbox but all versions of the ZF utilize output shaft speed for shift management. They only look at DSC signal/input for a "snow" mode that will allow you to rock out of a snow bank. living in Florida I gladly don't know anything about that. The DCTs have a "launch Mode" that must monitor the wheel speeds or receive input from the DSC to work correctly.

I'd try resetting the transmission adaptations and test again. Small changes in tire circumference should not make a noticeable change in the shifting.

[rothwem]

Originally Posted by rjahl

Don't know too much about the the GM gearbox but all versions of the ZF utilize output shaft speed for shift management. They only look at DSC signal/input for a "snow" mode that will allow you to rock out of a snow bank. living in Florida I gladly don't know anything about that. The DCTs have a "launch Mode" that must monitor the wheel speeds or receive input from the DSC to work correctly.

I'd try resetting the transmission adaptations and test again. Small changes in tire circumference should not make a noticeable change in the shifting.

Right, but I'm guessing there's a correlation programmed somewhere between the tail shaft speed and the vehicle speed, right? So if the tail shaft speed is saying that you're going 50, but you're actually going 53 because of the tire difference, then I would think shifting could be thrown off?

Also, if the trans goes off of the tailshaft, wouldn't there be a way to recalibrate the shifts if the rear axle ratio changes? I think the xDrive cars have a 3.91 diff while the RWD cars have a 3.46.

Anyways, I don't have the old tires anymore, I just have the stock sized tires.