Those shift points sound about right if your almost flooring it...It is [I]not/I] normal to be cruising around at 40MPH in 3rd, if that's what you're asking.

I am not sure if the '81 is all vacuum controlled, or if it has a bowden cable to adjust shift points. Could you pop the hood and look at the top of the valve cover to see if there's a cable?
The bowden cable is easily adjusted to allow for earlier shifts. If your transmission is vacuum controlled only, I'm sure someone else here can advise you on how to adjust those.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)