Also check for end wear:

https://www.stovebolt.com/ubbthreads/ubbthreads.php/ubb/showflat/Number/1003272/Searchpage/1/Main/148333/Words/"leaf+spring+wear"/Search/true/leaf-spring-wear.html#Post1003272


"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."