My H&R's didn't sag over time, they just initially settled lower than advertised. About a 2" front and 1.75" rear drop, versus advertised 1.75" front and 1.5" rear. I loved the springs. Great handling, good ride, perfect drop. I'd replace the shocks while you're at it....I went with KYB AGX.
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
Team B.O.B.® - Ballaz on a Budget