With your test, you are not accurately simulating the perches in the car. In the car, the perches are more "form-fitted" to the end of the spring. Here, the wires are sitting against flat surfaces. With low loads, the ends haven't quite yet taken the shape they will have at normal (much higher) loads.
This is not necessarily a bad thing... if we want others to be able to reproduce your test results, the experimental setup must be simple and easy enough for anyone else to duplicate.
However, to minimize this "end-deformation error", calculate the differential
in force it takes to deflect the spring one inch.
Example using your stock spring numbers:
0 inch -> 1 inch compression
0 lbs -> 121 lbs
Differential spring rate = 121 lbs - 0 lbs = 121 lbs/in
1 inch -> 2 inch compression
121 lbs -> 235 lbs
Differential spring rate = 235 lbs - 121 lbs = 114 lbs/in
Ideally, you would measure the differential spring rate at ride height; this would give the most meaningful results. E.g. if at ride height there is 4.3 inch of compression, measure the differential spring rate from 3.8 inches to 4.8 inches.
Unfortunately, you will probably reach the limit with your bathroom scale...
But the closer you can get to ride height, the better. 2 -> 3 inches would be better than 1 -> 2.
Remember to tare the weight of the spring before starting.
Another source of error I noticed: part of the spring is overhanging the cross-bar on the press, so it is supported on one side only. Therefore, it will want to bend like a banana. Try to center it within the cross-bar.
A note on safety: I would suggest passing a steel wire through some of the coils, as in extension springs for garage doors. If the ends slip, the spring will want to shoot sideways... which could be quite painful to someone! So a wire tied to the press could stop the spring from flying too far. However, if the wire gets pinched between the coils, the test results are invalid.
The fact that the spring is bending like a banana will tend to shoot the spring out.
[This message has been edited by pmbrunelle (edited 03-27-2017).]