There are quite a number of places on the internet to find the formula for calculating the rate for coil springs. A coil spring is primarily a torsional spring, wound up in a coil. As such wire diameter and number of coils has a big impact on the rate. On a 7/8 diameter spring, a single extra coil is adding about 2.75 inches to the length of the torsion bar, making it more supple. If you did the calculations it would probably surprise you the difference in spring rates (and this is assuming both springs are of the same material).

-Doug