It's more to do with the change in leverage than the weight (I'm guessing they're not that different weight-wise).
The upper rear wishbones are much shorter on the Saturn than the Sierra so the shocks have much less leverage and therefore need to be stronger to have the same effect.

Sort of - I ended up with 325lb springs as someone was selling some nearly new ones for a fraction of the price of a pair of new 300lb springs :o
I'm currently running 325lb on the front and 400 on the rear and I'm not in a rush to change them again.
Not saying they're the perfect spring rates for the Saturn build, or suggesting anyone else use them, but they seem to be working well for me at the moment.
I might drop both ends by 25 to 50 lbs next year though as it's possibly a tad too firm for some of the crappy roads I drive on :rolleyes:

Hi johno, Have you seen the magnetic camber gauge on EBay £20 ish don't know how to place link to it
Regards Mick.

Hi Mick,
I'll have a look around tonight on the bay of thieves...:)
I'm going to try and sort out how much caster I have but I bet that's awkward to measure:eek:
I reckon I've got nearly 9 degrees (eyed it with digital angle meter).