Specs for 2009 Z4 35i 18 inch Wheels/Tires

HI All -

I living in Tennessee and need to purchase a set of winter (cold weather) tires for my 2009 Z-4 35i/sports package. Car currently has 19 inch wheels with Bridgestone RFT Potenza high performance summer tires:

Front 225/35 R 19
Rear: 255/30R 19

Can anybody tell me the correct 18 inch wheel/tire specs for this car? I asked at the dealer and they acted like nobody ever asked this question before.

I am a novice to this but I understand I need to know the bolt pattern and offset (?) in order to purchase the correct wheel/tire package. I checked my owner's manual and do not see that level of detail provided.

Thanks very much in advance!

using an 18" OEM wheel, perhaps a Multispoke 293 or any other OEM Wheel the ET and the bolt pattern is still the same. Front ET is 29 and rear ET is 40 the same as if you were using 19" wheels.