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Brew, thanks for the detailed explanation. Makes sense to me. I was wondering why the rpms were so high on this truck in overdrive 2100 at 65 mph. The kid I bought it from put 33s on and did not change the gearing.

Going to a larger diameter tire without re-gearing will reduce your engine rpms at any given speed (unless it keeps it out of o/d). It's like changing your rear end from 4.10 to 3.90. Perhaps the rpms seem higher because you're actually doing 70 instead of 65.

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Now I need to check the diffs and see if I can read the tag for the gear ratio, although I think the spec is 3.73 for the Ford 10.5" rear. Guess I need to get taller gears and some lockers while I'm at it

You want just the opposite ("shorter" gears; higher numerical ratio) if you wish to restore stock performance, and speedo accuracy.

1997 T 100 4X4 ExtraCab
5 speed, White
Worthless factory bedliner
265/75/16 Firestone Destination A/Ts
Aisin Manual Hubs
90k miles