The specific rate springs are barely noticeable (to me at least) in NVH.
I had the Eibach 450-580# and went to 650# linears. Hardly a notice.
A H-U-G-E difference (worse) was the 45-series tires. I went from 245-50-16 Gatobacks to 245-45-16 A008RS-II (R1-type).
These tires are STIFF!!! I could feel if I drove over a penny!
Now I have 275-40-17's on 17x9-1/2" wheels
with g-Force R1s. These are noticeably stiff, but maybe its the taller tire that helped a bit in the road noise.
Back to the springs... I recommend a variable rate spring for a daily driver. If its only a weekend warrior, or play car, then go ahead with the specific rates.
Thats my story, and I'm stickin' to it ;-)
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Darius Rudis
Home built t-arm/panhard suspension powered by an S-trimmed motor :-)
My Mustang Page