at what rpm should engine run at 55mph out of tow/haul mode
It depends on the gear ratio of the rear end. Put the transmission in tow mode and see what the RPMs are at 55 mph. Take it out of tow mode and now see if the RPMs have dropped. If the RPMs have not dropped, that may be the problem. Since 55 mph is around the normal highway driving speed, I would guess the engine should be turning at or near the 1800 to 2200 RPM range. The 70s trucks would turn around 3000 to 3500 RPM range due to the rear end and transmission gearing.
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