looking it up all we can find is that the actual official standard (which most records deviated from because it was standardized after they were already in production) was 78.26 rpm for the simple reason that that was the speed a common 3600 rpm motor (the speed of a synchronous motor on 60 Hz AC) would give you with a cheap and common 46:1 gearing
but they could have gotten an even 80 if they'd just used a 45:1 gearing so why didn't they
-F
@Felthry Wikipedia seems to suggest that RPMs were originally all over the place, but approximately-78 got chosen because speeds were calibrated with stroboscopic discs: https://en.wikipedia.org/wiki/Phonograph_record#78_rpm_disc_developments
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