Over the years, I've noticed that lots of people are confused about the effect of gear ratios of diesel locomotives. Just for info, here are the basics.

1: A traction motor, for various models, has a limit on the RPM that it can operate at.
A high gear ratio allows for a higher maximum speed. On EMD 62:15 used to be about
65 mph, 59:18 would allow 103 mph. I think ATSF E-units had the highest available.

2: Because of the gear tooth pitch and the related mathemeatics, the two numbers in the gear
ratio add up to the same number (old EMD = 77); 62:15, 59:18, etc.

3: The traction motor can only operate at high amperage, slow RPM at a slow speed for a limited
amount of time. This is the "short term rating", which on GP's & F's would be 15 mph
for about 15
minutes. A lower gear ratio/lower top speed allows a longer maximum power period.
Traction motor blowers help cool the motors, but at high amperage conditions, the heat
can still build up to the point of melting insulation materials.

This is one reason (also the idler axle doesn't do any work) why passenger units
such as as E-units did not see much use in
freight service, they can't run at slower speeds at full power. PC used E's on
piggyback trains, and EL used PA's on some priority freights for a while.


4: At any gear ratio, the motor has enough torque to slip the wheels. Therefore, only the
weight on drivers and adhesion determines the tractive effort. Gear ratio does not
effect it.

5: As noted above, the differences in the acceleration rates for different gear ratios is
small.

My question is what railroads, if any, used EMD's lowest ratio 65:12 (58 mph) on road units, maybe in drag pr helper service?