Can someone explain to me how to measure offset for wheel spacers?

I have 13 Cayenne with 20" Spider wheels and would like to add spacers. Not sure how to measure properly and what to get. Any help greatly appreciated!

Any pictures if anyone had it done?

 

I never measured the Cayenne for spacers but you can use the same technique i used for my M3. Get a piece of string 1' long, tape, a quarter, and a ruler. Tape one end of the string to the quarter and tape the other end to the top edge of the fender. Use the ruler to measure the wheel inset at its top edge. You should be able to get a good idea of what sized spacers to use from here.

An example. My M3 has a 29mm wheel inset in front and a 14mm wheel inset in back. I got a 25mm spacer for the front and 10mm spacer for the rear to give it that aggressive stance.

 

Here is some reference including some pics with my 20" Spyder wheels. They are for sale as well.

https://www.6speedonline.com/forums/...m-spacers.html