Sounds like a variable rate spring, which gets a bit harder to work out, and the wire diameter and number of coils would tally with this one.
RKB101111
http://members.shaw.ca/jbarge/RKB101111.jpg
Which is a 110/130 OE rear spring from VIN 159807 with a rate of 270/420lb/in according to JB.
The formula I've always used (and tallies with Land Rover spring rates) is
K=Gd(to the powerof4)/64R(to the power of 3)N
Where K=spring rate (pounds)
G= modulus of rigidity which for most spring steels is 1500000
d= wire diameter in inches
R= mean radius of spring
N= number of active coils
To work out the number of active coils is the total number of coils less...
Squared and ground ends = -1.75 (Land Rover and FWIW coil over race car springs)
plain ends = -0.5
plain ends ground = -1.0

