Thanks! I'm think you're right. The Sport Package for the Z4 2.5i contained 17-inch wheels (at least the US spec). The car is a Euro spec and the first owner of the car was an employee of BMW AG, so the car might have been upgraded pre-delivery. I do have every invoice for the car regarding service and maintenance since delivery. However I do not have an invoice for the upgrade to 18-inch wheels, so I´m not sure of they came with the car or if they where replaced.
2003 Z4 2.5i manual, sterling gray/black oregon leather/aluminium, sport package, stock
2012 Volvo XC60 D3 AWD Polestar (daily driver)
2013 Volvo V60 D4 AWD (hers)