I did do a search, and maybe i didn't use the right search string but I wanted to know how far off can the diameters be without causing an issue? Will .5" of overall rolling diameter cause an issue, is there a threshold?

 

Should not matter.
I run a couple sets, my primary set is 19 inch wheels with 275/30 on the rear and 245/35 on the front. Overall diameter is 25.5 inches.
My winter set are the factory 18 inch wheels with 255/40 on the rear and 225/40 on the front with winter tires. Not sure the overall diameter of my winter set but I know they are different, by a half inch or more.
I don't have any issues with either set or sizes, other than the 19's are harsh, I need to check tire pressure every week because they always look flat, and I avoid potholes like the plague.
Probably more info than you needed, hope it helps.

 

This runs contrary to all the other internet wisdom on x-drive tire sizes. But internet wisdom is worth what you pay for it.

Your summer set is within 1% front to back, and your winter set is 3.6% delta.

 

+1

I bought my wheels from Modbargains and this was the advice I was given for my 2010 535i xdrive:

"I can tell you from firsthand experience that I’ve sold and installed tons of wheels to other E60 Xi models and you can definitely go wider and larger. I’ve done as wide as a 10” width out back for these guys without rubbing issues and keeping the rolling diameter within 1% for the AWD model. Trust me on this, I wouldn’t lead in you in the wrong direction and this is all based off experience."

 

I should caveat that my car is sans xdrive. A wheel/tire combo that is .5" larger is fine as long as the same diameter is the maintained across all four wheels/tires. To your point, my winter set does have a large diameter difference front to back (but that is what was available to me in snow tires). I would NOT recommend this set-up for an xdrive. Overall diameter/rolling circumference should be the same for all wheels/tires on an xdrive. Good to bring that up, Audiphool.

 

My last AWD there was a 2% allowance, so this AWD in the BMW would have an issue with the 1.2% difference in circumference?

 

I would say 1% would be just fine. There has to be some tolerance just for differences in wear F/R.

 

That is my belief, now is there coding that needs to be done?

 

No that I've ever seen.

 

No coding at all. Just put the wheels on and enjoy. I have a set of 18" matte black staggered Forgestar F14's for sale in the classified section that were made for my 2010 535i xdrive, so same as your car.