Dave's advice is sound.
I found that I drove more "by ear" (the sound of the engine) rather than much else.
I only shifted out of overdrive when I was actually climbing some sort of grade. (The slight grades of overpasses don't count.)
I pull/pulled either a Dodge Dakota pickup or a Jeep Grand Cherokee, with the Dakota weighing at about 4500 lbs.
I had the very early Ford V-10 (with only 275 HP and the clunker Ford 4-speed tranny), and it pulled the toads with no problem. Now that I have the much newer V-10 (with 362 HP and the very smooth 5-speed Ford "Allison-work-alike" tranny), I have no problems at all.