I know this is an old thread but just in case someone non-technically minded finds it in a search, I'd like to correct a misunderstanding. The rear spring rate on a '79-'04 Mustang is so low compared with the fronts for two reasons. A - the rear roll centre is ridiculously high (16") because it is set by the two UCAs. The higher the rear roll centre, the lower the spring rate B - The stock Mustang 4-link has terrible binding under articulation. This means the control arm bushings effectively act as springs and add to the overall rate. This binding is also why the stock mustang rear end has such bleeding awful handling characteristics and was replaced by a 3-link in the S197. If the live axle '79-'04 Mustang is fitted with a bind-free rear suspension with a lower rear roll centre, whether a Torque Arm (Griggs, MM, GM F-body) or a 3-Link (EVM, S197, NASCAR, etc), then the spring rate balance will change. A stock spring will be way too soft and the back end will hop like crazy. For example: Stock '87-'93: 425-530 Front, 200-300 Rear '87-'93 with TA/3-link: 425-530 Front, 325-430 Rear (ish) If you look at an S197 ('05-'13), you'll see spring rates that are much more balanced, similar to the TA/3-link example above (although much lower overall due to having a much higher wheel rate in general).