And while we're on the subject.

RPM already plural, so there is no such thing as RPMs.  Let me explain:

RPM - Revolutions per minute.  See, "revolutions" is already a plural noun,
so there is no need  to say "750 RPMs," but you may say "750 RPM."  Because
you would never say "750 revolutions per minutes," but you do say "750
revolutions per minute."

In short:

750 RPM - correct
750 RPMs - incorrect
750 RPM's - also incorrect
750 RPM - correct

Thank you, that's all for now.  Next week we discuss RPM's close cousin,
PMS.  Class dismissed.

Chris.