[racer]

Sounds like you have an "early" car and want to fit "late" wheels.

So.. you need a spacer to go from 52 et minus 23mm et and from 47mm et to a 23mm et so... you need 29mm front and 24mm spacer rear. They neededn't be perfect, though, since as you look at the 944 fenders, there is quite a bit of room.

1" is about 25.4mm

This "upgrade" has been done many times.

Offset (ET) is the difference between the mounting face of the wheel and the actual wheel centerline.