How Much Drop You're Getting, and What Changes With It
A set of lowering springs typically drops a Mustang by 1 to 1.5 inches, though the exact amount depends on the spring rate and whether the front and rear rates are matched. That drop comes out of your suspension's usable travel, so the shorter your springs compress the car, the less room your factory struts and shocks have left to do their job before bottoming out.
Springs alone work fine on struts and shocks that are still in good shape and rated for the reduced travel, which is most of what a moderate drop asks of them. Push past that range, or pair springs with shocks valved for stock ride height, and the ride becomes harsh, or the struts wear out faster than they should. Some owners also notice the car settling unevenly over the first few hundred miles as a progressive-rate spring seats in, which is normal and not a sign of a bad install. That's the point where a shock and spring kit or a full lowering kit matched to the new ride height makes more sense than springs by themselves.
