4/6 on 15's
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4/6 on 20's
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2/4 on 15's
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2/4 on 20's
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And my sig is 4/7 on 20's.
My opinion, just from owning trucks with both drops, is go 4/6! I wish that I had done that first to my '97 (the blue one). The only reason that I haven't is because of time constraints, & for the fact that I am not looking forward to re-doing the front spring hangers. A major PITA!!! When it does get the 4/6, I will be cutting the front coils, due to the increased spring rate. 2/4 is not enough! You'll want to go lower after 1 season. Really! You will!
One other thing, though. On a 4/6, I recommend a small bed crossmember notch.