Pajama Johnson

I have a tire/wheel fitment/size issue with my 2011 F07 550i GT xDrive

I purchased the 312 style wheels with the following specs:

front: 21 x 8.5
rear: 21 x 10

I purchased tires (245/40/21 and 265/40/21) for the wheels, but they are apparently the wrong size. The car now drives terrible. The best description I can offer is what Mark from Just German Automotive explained as being like "on a beach ball." The car used to run tight and straight, but now it's all over the road when turning or changing lanes quickly.

It's been explained to me that the respective sizes are too large as well as too different from one and other in terms of aspect ration. It's been explained that part of the handling problem is because it's an all wheel drive (xDrive) vehicle and that the different rolling diameter for the front and back are adding to the problem.

I've done enough checking around ... these wheels ARE MADE FOR the 5-series and the 7-series, so all of the people who have told me that they just won't work are wrong. I've corresponded on BMW forums with 2 other people with the same vehicle and the same wheels and they said it handles fine ... although they both told me different sizes.

My problem is that I can't afford to keep throwing tires at this, and none of the tire shops are willing to take responsibility for guaranteeing the tire size I need to solve the handling problem.

One suggestion was:
front: 245/30/21
rear: 265/30/21

Another suggestion was:
front: 245/40/21
rear: 255/35/21

I understand that calibration is an issue, especially with xDrive and that having mismatched diameters between front and back can tear up the drive train, or so I've been told. I also understand that not everyone is going to agree with the styling and aesthetics of larger wheels. However, I know that at least 3 other people with the same GT have told me on the 5GT FB group that they have zero issues with 21's ... I just need to know the right tire sizes, and don't have the time or inclination to become a tire expert. Just need someone to give me some solid guidance on what tire sizes to choose!

Thanks in advance :-)

-Patrick