[LUKEDAVIE]

I installed new wheels and tires on my 2011 x3 28i xDrive. They are 19inch TSW Nurenburgring wheels, the back one is slightly larger in width than the front. The wheels came off a 2003 m3 from Facebook marketplace. While putting new tires on to see what looks best fitment-wise, we ended up putting the front tire slightly smaller than the back. The car drove great but noticed traction control engaging while accelerating at speeds over 60mph. It didn't always do this but if I turned the traction control off I would not have a problem so I disregarded of this happening. A few months down the road I noticed the car shifting weirdly. It kinda jults when stopping or accelerating like when it needs to shift gears. This turned into vibrations while driving as well. If I drive with traction control off and shift manually it helps it not do this as much but I can definitely still feel it. Machanic said he thinks it might be the wheels being different sizes and need to further look into that. Has anyone got advice for me? Is it the wheels or is my transmission just messed up or maybe a combination of both?

[pungo]

Welcome to the forum....
I assume you have xdrive and your front and rear tires are not the same diameter. If so, you probably screwed up the transfer case driving it like that for a few months
You can have different size tires front to rear as long as the diameter remains the same within 1% of each other.

[MSport_Jon]

Like Pungo said, You cannot have an asymmetrical setup unless you can bring it to 1% or less total diameter with each axle. Sounds like this is the case you are having.

[Opie55]

You don't say how much the diameter differs front and rear, but as the others have said it can't be much. Your car thinks some of your wheels are spinning because they are not rotating at the same rate. It tries to compensate but it can only do so much for so long. You need to get different wheels and/or tires asap to get them the same diameter and see how things go, but likely you fried your transfer case. Between that and the wheels/tires you are looking at a chunk of change.

[Opie55]

And just FYI in case you don't know what a transfer case is, on four wheel drive cars there has to be a way to get the single output at the back of the transmission split into two outputs so power can be delivered to all four wheels. It's an oddly shaped aluminum case full of gears and clutches bolted to the back of the transmission, and has two output flanges - one for the rear driveshaft and one for the front. It is oddly shaped because the driveshaft going towards the front wheels has to physically clear the transmission and engine.

[3002 tii]

Theres a chance overall diameter is still same front to back. Wider doesn’t mean taller if he ran the correct tire profile. But it the tire profile is same front to back, yea you probably messed up the xfer case.