With our rig, now at a bit over 260,000 miles, it has always shifted at the lower mph but when in Tow/Haul mode it holds back to shift into 6th gear until around 65-58 mph. Some years ago, I replaced the tranny fluid with the Alison Transynd full synthetic and it still shifts great but still shifts at the higher speed into 6th gear. I do a drain and fill of the tranny fluid every couple of years, and replace the spin on filter every 30,000 miles.
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Bob & Becky
2012 3402RL
2012 Chevy 2500HD D/A CC 4WD
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