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I guess you are talking about 0.4mm nozzle (trace width). Although your volumetric calculation is right, there is one slight problem - maximal turning speed of the motor. On the output shaft 28BYJ-48 can do only 15RPM.
The only way you can achieve 60mm/s print speed with 0.4mm nozzle a 0.31mm thick layer on ToyREP extruder is with 3mm filament as you will achieve desired volume at 14RPM (1.75mm filament will need almost 40RPM!). For the extruder is 1.18mm of filament per second the speed limit.