Like you, I moved up to the 2500 Hemi. I've towed 8400# with the 8 speed 1500 with 3.92 gears and it was pushing it. I have driven the 1500 8 speed with 3.21 gears, but did not tow with it
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I towed 8400# with the 2500 with the 3.73 gears and the 6 speed. I thought it liked the lower gears...