[Antetokounmpo]

So here's my dilemma, I run a set of 18x8 and 18x9 staggered wheels on my 08 xi and currently have 225/40/18 and 265/35/18 Conti DW tires mounted on them. They're 1.008% off from one another and every time I change gears there's a slight vibration from the drivetrain that lasts about 2 seconds then goes away then comes back when I shift again. This doesn't happen on my non-staggered winter wheels so I'm assuming it's because of the diameter difference. However, BMW recommends 225/40/18 and 255/35/18 and these are 1.004% off from one another, now I know the magic number is 1% and I'm almost identical to what BMW recommends so I'm wondering why I get this weird vibration when I shift.

Part II: Pretty soon I'm gonna need some new rubber, I want to go as square as possible because I do some casual autocrossing and the staggered setup pushes a lot unless you can trail brake really well. The DW's are okay, I've got about 8500 miles on them and they're just about at the wear marks. I'm pretty set on Michelin Pilot Super Sports and I'm trying to decide which sizes to get. Like I said I want square if possible. The only way I could see that working is a 235/40/18 setup on all four corners. Yes, I know my wheels are staggered and the rears will still be wider but it's as close as I can get. I'm just wondering if the 235s will be too wide for the 8'' wheel upfront and too narrow for the 9'' wheel in the rear. I know they'll fit but I don't want them too stretched or bulged where they don't perform like the should. Tire rack does say the 235/40/18 PSS can be mounted on an 8''-9.5'' wheel so I'm just a little worried about being right on the edge. I don't think any other size will work with xDrive besides 225/255 like BMW says (I know a 235/35/18 and 265/35/18 will work too but I don't want staggered if possible). If anybody has run this setup please chime in. Thanks!

EDIT: I took a look at the revolutions per mile of the DW's in the size I run and they're only off by 3 per mile which is identical to the OE potenza runflat in a 225/255 configuration. Not sure if it really matters but it seems like my tires are the same as OE in terms of diameter so I'm wondering why I'm getting the vibration.