Just bought a 2003 Z4 2.5 with factory 16" wheels. Since I'm new at the Z4 game I'm not sure which later wheels will fit thiis car without modifications. I'm looking for a set of 4 used 18" OEM wheels and tires to replace the originals. Will the 18" wheels and tires work without changing anything on the car? Where's a good spot to find these used? All help appreciated.

The 18's ought to fit just fine, as the Z4 comes with that size in the current sport package.

Look for bargains (?) on the Tire and Wheel Forum section.

If the fit the E46 they most likely will fit the Z4.

Just bought a 2003 Z4 2.5 with factory 16" wheels. Since I'm new at the Z4 game I'm not sure which later wheels will fit thiis car without modifications. I'm looking for a set of 4 used 18" OEM wheels and tires to replace the originals. Will the 18" wheels and tires work without changing anything on the car? Where's a good spot to find these used? All help appreciated.

They'll work - my car came with the 16" wheels and I bumped up to the 18" in my sig pic when I changed from RFTs to Goodyears F1s. No issues, and that was 2 years ago. Currently both my cars have 18" ZHP reps.

Are the 18" wheels all the same or staggered?

I want to upgrade my 2.5 to a 18" OEM wheel as well and was wondering if the larger wheels with the additional weight significantly reduces performance?

Are the 18" wheels all the same or staggered?

Mine are staggered 225 fronts 255 rears.

I want to upgrade my 2.5 to a 18" OEM wheel as well and was wondering if the larger wheels with the additional weight significantly reduces performance?

Nope, because the run-flats are heavier than standard tires you tend to break even in terms of performance, and you come out way ahead in handling and comfort.