I really like the way M3 wheels look on that car. I belive it is 225/45/18 front and 255/40/18 back.

I currently have sport package on with quattro, but planning next spring to put (taking the diamiter in mind):

235/45 17" front (Michelin Primacy Sport)
- planning to use them from my sport package)
255/40 18" back
- get different tires for the back. Of course I understand they have to similar drivablility as
Michelins including traction.
(I know 255 will fit whitout any issues, I have 2 friends that use that applications without any rubbing even with full load in the back)

I think it would look very phat, bullish, M3sh.

My concern is how this will efect QUATTRO or ESP, what about tires, should I get Michelins like in the front or get smth different for the back ?
Will my drive really be effected.

Keep in mind I am not going to participate in any drag or autoX races. It is strickly for daily driving and looks.

I have seen many applications like that on old generations Audis without problems.

I need your input !!
Thanks

In my humble opinion I don't think this is such a good idea. First off, you should not use different tires than what you have in the front, and if the front ones are not new then you should be 4 new tires. That's for any car with all wheel drive...As for the bigger tires in back, quattro and esp rely on the fact that the contact patches are the same size to calculate how to regulate power, so IMO I think the bigger rear setup might mess with something...
This is just what I think, sorry that's all you get on a Friday morning....:)

BMW claims their tires don't need to be rotated (or at least that they don't need to be rotated front to back) because of the equal weight distribution, the rear wheel drive with front wheel steering, etc.

However, us AWD people still get some more wear up front from the steering and such, so you'd need to rotate front to back or else suffer from faster wear on those expensive tires you're considering.

will screw with the spedometer, ride height will be affected b/c you will have a greater diameter wheel/tire combo on the rear.

Talk to a good tire/wheel company like Tire Rack. They will help you pick an appropriately sized wheel and tire combo.

Dont worry about diamiter this can be checked, I am mostly concerned about the QUATTRO and tire witdth issue.