The coils offer a smoother ride. The 3500 still has leaf springs but is also available with the new air bag setup in the rear.
From Diesel Power Mag:
Taking cues from the 1500, a standard five-link coil rear suspension has been added to the 2500 to deliver best-in-class ride. Compared to a leaf spring system, this rear suspension will provide better articulation over obstacles. The robust coil springs handle up to 17,940 pounds for towing. An optional rear air suspension system is available, where an air bag replaces the coil spring, much like the Ram 1500. Load capacity is not sacrificed, and in fact, ride and handling get even better. The air suspension is also capable of load-leveling, where the load on the rear suspension from a trailer or payload is automatically detected and accounted for.
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