As Jack said, the Wind resistance makes such a huge difference... that's the one big reason I am not a fan of 5th wheel Trailers... they give you this huge air brake the whole time you are towing.
Also its really not possible for the rpm to be different at the same speed if the TCC is locked up... what differs is the amount of fuel needed to maintain that RPM at any given point
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DJ Phoenix (my modded) Hi-1200/240Tow/75FS/80DD/100/140SS +Minotaur
ASE Master Auto Technician 
2001 Ford Excursion 7.3 4x4
Projects. 1970's Hustler and Max II Amphibious ATV's
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