Look, I'm not sure if I agree with all this business about spring rates being the same across all cars.
Sure, on mcpherson strut cars it's going to be reasonably similar, but on a wishbone setup, it would be nearly impossible to compare spring rates across different platforms.
Someone please correct me if I'm wrong, but everyone seems to be leaving out of the equation the location of the springs on the control arms. A higher spring rate mounted further in on an arm could be compared to a lower spring rate mounted further out on an arm. The spring itself might compress 1mm for every 7 kg, but with different length arms this could translate any number of ways.
Toyota Soarers, Chasers etc, as well as FD RX7's typically run a stiffer spring rate than say an S15 or S14.
SK, i'd be interested to hear your theories on this.