Small, permanent magnet, brushed tri-phase DC motors ('toy motors') may give a speed/torque curve, or at minimum a no load RPM. What is typical RPM accuracy relative to the datasheet, and how precise is it's RPM (i.e. its variation) from run to run? Moreover what variables affect this accuracy/precision?
I have not gotten to the point of measuring RPMs with a tachometer or encoder, but get the sense that even under no-load, forces from gravity affect the speed/current consumption when the motor is positioned in different angles or moved with acceleration. Beyond external forces, it seems like manufacturing variances in coil turning, permanent magnet strength as well as inconsistent contact between the brushes and commutator could all cause RPM inconsistencies for a given voltage/current. I took apart a Mabuchi 130-form factor motor and manually counted the same number of windings (70, specifically) on each armature, which was good to see. However, I expect that I will need to implement closed-loop control to achieve constant RPM.