That is the rating of the spring. Likely as manufactured. Leaf springs a naturally progressive (the farther you get into their "travel", the stiffer they get), thus the "real" rates are slightly higher than the theoretical in the original post. I don't have the rate curve handy, but we may be able to come close if we really wanted to pick nits. Gunk between leaves, and in shackles will add to he ride stiffnes, too. A lot of variables, here.
"other peoples junk, is something or other" - Militarypotts 02/07/2011