I had the VW/Eibach springs, which are known commonly to be a little less stiff than ProKit (though I dont really believe they're different). If you look at the Eibach part numbers, ProKit correspond to the last column. Here is the spec sheet that came with them.
Front: OEM thickness was 12.5mm, this is 12.75mm, but there's one more coil. Anyway, the spec sheet says 8% stiffer wire, 7 coils not 6, should be 7.5% less stiff (should be).
Rear: OEM thickness was 12.0mm, this is 12.5mm, but there are 2 more coils, Anyway, the spec sheet says 17.75% stiffer steel, 10 coils not 8, should be 5.8% less stiff (should be).
Have a look here
http://forums.vwvortex.com/zerothread?id=1586160
