Here's what Chevy(and Zora) thought was best for the IRS Corvette rear springs...

Standard spring rates:
63... 192#
64-74...140# (1st stage) 196#(2nd stage)
75...196#
76-77...196#

Heavy duty:
63-77...305#
78-79...307#
80-82...266#

There are differences in a lot of the steel springs, as far as how many leaves they have, the design load(weight), and thickness of the leaves.
The 64-74 standard spring is the only one that is a "two stage" spring, meaning it is rated at two different levels of compression. The higher rating(196#) is with the spring almost 4 inches past "flat"!!! All the other springs maintain the nominal rating thru-out their travel.

So...a 300# spring would have the same rating(approx) as the factory HP suspension. This would include the Z07, and the "Gymkahana" set-ups.

This is one reason I always question the logic of using some of the aftermarket, 340#-370# springs...
Granted, tire design has come a long way since then, but...Adams' Apple2007-03-15 20:58:20

Very helpful - thanks for the info Joel!

I just can't understand someone wanting to make their car ride like a wagon...but, then again, I am an old fart. I like to be comfortable when cruizin...

I see where the 78/79 gymkhana spring rates are the stiffest. I'll vouch for that! It can be a teeth jarring ride as is, much less going stiffer. Then again, I'm an old fart too! I don't even like LOUD exhaust.  Mike