Well I decided to email ARB USA and ask about the deal on these OME springs. Here is what I got:
Andrew-
Our R50 Pathfinder springs all have the same spring rate of 200 lbs/in while the OE springs are 150 lbs/in. As our application page shows, each spring is designed around a vehicle with a certain amount of extra weight. As you step from light load, to medium, and heavy there is a 10mm height change in the coil itself. The extra height of the spring counteracts the additional weight so the end result is .50” of lift when the weight is matched to the appropriate coil.
If you are only planning on running skid plates, I would suggest the Light Load springs (2921) in the front and the Medium Load (2922) for the rear.
Hope this helps!
Matt Glass
ARB USA
Tech and Marketing Assistant
So it seems that you really DO get more height with the "heavier" springs and they are all the same stiffness! 10mm is about 1/2 inch so light springs are .5 lift, med are 1", and heavy are 1.5" lift. And the rear springs are only available as med at 1" lift. And remember don't get confused about the Rocky road website, their listings of what is medium and what is heavy is incorrect. They only sell light and medium but they call them medium and heavy.
I am going to email ARB in austrailia to make sure what this guy says is correct. Too confusing!