The early GMs were publicised when they were new as being capable of 90 mph (~ 145 km/h).
The ELs were supposedly good for 140 km/h (?).
Is this solely to do with the gear ratios? I know that the ELs were re-geared but don't know about the GMs (early or late).
The gear ratios directly affect the rotational speed of the motors at a given track speed and are relevant.
DC motors have complex windings on the armature held in place by wire wound arond the armature.
The windings are soldered to connecting cables to the commutator which is made up of tapered copper pieces held in place also by wire wrapping. If the securing wires break, all sorts of expensive pieces of metal come loose and can lock the motor solid.
The gear ratio is supposed to keep the rotational speed of the motor at a safe level at top permitted speed.
I don't know if all the remaining GM class have their original gear ratios.
Like YM I haven't heard of a change.
The Freight Australia/PN A class did get a lower speed gear ratio when the traction motors were replaced, but it is understood that the four Vline locomotives kept a high speed ratio. This is pretty theoretical now....
I'm tol the AN class were regeared by PN to more closely match the NR class since the two types tend to run together.
The AN class have bigger motors than the NR and with the right ratio provide more assistance on the NSW main south.