I bought my 996TT with aftermarket 18"x10" rear wheels of unknown origin. Offset is 47mm (I have couble checked) but the wheels look to sit too far inboard, with a gap between the fender lip and the wheel. 47 mm iss close to stock. What am I missing? The picture above shows the car with stock wheels, which is what I normally use on the track.

The center caps for the wheels are diameter 75mm but the "hooks" that grip the rim are shorter than the ones on stock wheel center caps. I measure appr. 4.5mm on the aftermarket wheels vs. 8 mm on stock wheel caps. I am looking to find replacement center caps since I lost a couple when the brakes got hot on the track. What are these center caps for, and where should I look?