[Formulabruce]

Originally Posted by 4birds

Also, Eaton springs chart shows a definite difference between the left and right rear leaf springs for the 1972. Not sure if the spring rate was different or the arch was different between the two sides.

Pontiac Firebird DS spring "rate" was determined at a greater arc than the Pass side at the same deflection due to higher perceived Driver weight.
Note. This was PRE "Cross slope" paving ( crown in middle of road), and Pre Radial Tire, and these cars had a Straight Upper "A" arm mount in front , which got changed for crowned roads to an offset for this reason. At the same time Pontiac made the Rate at the same deflection as the other side in mid 1972 Calendar year. This made both rear springs identical by 73 production. The difference was about 15 lbs at a greater arch, not much, and therefore Pontiac allowed substitution till they ran out, just like a parts change pick order, except for the Sub assembly area. ( hope this wasnt too big a side track of this thread)