Determining wheel size
 

I have a set of 15" wheels on my 83' SC. I believe they are 7's & 8's. How can I confirm that I am correct on that assumption?

^
Measure them.....

if they're fuchs, they should have Porsche part numbers (and size markings) on the insides of the spokes -- other wheels (even aftermarket) should have similar markings -- pull a front and rear wheel and look what they show from the inside

AG81 -- good point, but seems that would be difficult to accurately do if they already have tires mounted --

Back of the spokes

If they are Fuchs
http://forums.pelicanparts.com/uploa...1587591628.jpg

http://forums.pelicanparts.com/uploa...1587590901.jpg

measure if not

this is a 7
http://forums.pelicanparts.com/uploa...1587590982.jpg

this is a 6
http://forums.pelicanparts.com/uploa...1587590982.jpg

wheels are defined by the bead width not overall width
http://forums.pelicanparts.com/uploa...1587590982.jpg

If tires are mounted use rubber bands to hold 2 laths that rest on the wheel flanges, measure the distance between laths and then subtract the width of the 2 flanges, for Fuchs you would subtract .7, most modern wheels have .5" flanges so subtract 1", BBS is an exception subtract .8
http://forums.pelicanparts.com/uploa...1587590982.jpg