Incorrect. The bar is 'tempered'. How come you can get springs in any rate you want made from exactly the same size wire (or Bar), in the same diameter AND at the same length? Ever heard of ADJUSTABLE COILOVERS?? It has nothing to do with the shocks - a shocks rebound rate is not determined by...
Errrm, not wanting to point out the bleedin' obvious BUT:
When Nissan make their springs (or others with aftermarket ones), what do they do to determine the size before putting into the car?
THEY CUT THEM
Every spring ever made is 'cut' once at the appropriate length for the desired...