I collected roughly 1,000 index pulses from the Sony MPF920-E with one common floppy inserted.
The motor speed variation was very very small. I am impressed as to the accuracy.
Across 967 index pulses, all were within 56 microseconds of each other. The average was 200.487ms. Most of the group were within 20 microseconds of the average.
The standard deviation is 9.46 microseconds.