Yeah a couple things to take into consideration when you talk about lowering springs ..
Where did you get these Sprint springs first of all ?? Did they claim to be 1.5" lowering springs, or are you just making the assumption based on the unsprung height of the springs side by side ?
This height has nothing to do with the actual amount of drop you will get when you install them into the car. The SC springs are pretty stiff, and have a different spring rate than other aftermarket springs. These Sprint springs may be shorter, but stiffer, and not compress as much as the SC springs when installed .. so going by unsprung height alone doesnt mean anything.
Brand new springs may take a little time to settle in, so after a couple weeks, they may be lower than what they are when you first install them.
Your SC springs are lower than "stock" springs, on an LX Tbird, Im not sure exactly how much, either 1/2 or 1" lower already, so a 1.5" drop spring isnt going to be that much lower when you install them on your SC as it would be on an LX - the amount of drop is an approximation, and most of the springs are made for V8 Tbirds, which do have a different weight on the front end of the car than the SC - how much I dont know either.
Tire size is another factor, like Dave mentioned above .. does your SC have stock size 16" tires and you have 17" on the one with the lowering springs ? Did you take a before and after measurement on the same vehicle you installed the springs on, or just compare the height with the other car after the installation ?
- Dan