FMS C springs (Part No. M5300C) have a spring rate of 650 lb/in front and 200-300 lb/in rear. They are similar to Steeda Sport springs (650 front and 200-250 rear) and Eibach Sport springs (660 front and 260 rear).   Most sources I've found list the OEM V6 springs as being 400 lb/in front and 190 lb/in rear, which is close to the numbers I got when I measured and calculated the rates of the springs that came off of my car.   You can see those calculations and a table of Mustang spring rates here. The FMS catalog says that C springs will lower a 5.0L Mustang 7/8" in the front, and 1/2" in the rear.   That's for 1979-1993 (Fox) Mustangs.   The C springs initially dropped my 2000 convertible 1-1/2" in front and 5/8" in the rear.  Cutting the dead coil off the rear springs increased the rear drop to one inch.   (They can be cut by hand using a hacksaw with a "carbide grit" blade)   Adding one set of Kenny Brown spring spacers in front reduced the front drop to one inch.   The car is now level, dropped one inch front and back.   Here are the measurements that I made: I installed new urethane spring isolators and a new pinion snubber (rear axle bumpstop) when I replaced the springs.   They weren't really necessary.   The stock isolators were in good shape and could have been reused.   I could have cut down the stock bumpstop with a hacksaw. I haven't written up the spring installation process, because that's already been done by others.   Here are links to three of those "how-to" articles:    springs 1   springs 2   springs 3   Bilstein shocks/struts, Maximum Motorsports caster/camber plates, and upgraded tires. You will find a "how to" article on the installation of shocks and struts here.   Links to installation instructions for the caster/camber plates are posted on the Maximum Motorsports web site. Last updated: 6 May 2003