I know, but not that many cars had tachs then, at least not ones I drove. You could also time yourself between mileposts if you were on a highway that had them, otherwise it was guesswork. Anyway, it was more about just going as fast as you could, not so much the actual mph number.
Tachs were often optional equipment back then. I always thought, if you’re gonna save money by not getting the automatic, at least use some of the money you saved and get the tach.
In the worst case you didn’t even get a temperature gauge, just an idiot light, unless you got the optional gauge cluster (which also included a tach).