Presumably you did this with the wheel off the floor under no load. So the controller will be running the motor up to it's speed limit and then cutting the power to maintain that speed so there's just enough power to overcome friction at that speed. It's presumably doing this by cutting the duty cycle so that the current drops to just above zero. I'd expect a smaller duty cycle than you found to do this, but maybe the back emf and speed means torque has already dropped off considerably so ~50% duty cycle is needed.I decided to dig out the oscilloscope today and have a look at the drive signal on one of the motor phases. What I found was interesting and not what I was expecting.
As I mentioned above the three motor phases are driven by a pulse width modulated signal and what I was expecting to see was a noticeable variation in the duty cycle of the signal and perhaps variation in the pulse amplitude, however, between the three modes there was only a small variation in duty cycle and the amplitude was the same between modes.
I checked at full speed in each mode and found going from low to medium to high the duty cycle was: 50, 55 and 60%. The pulse amplitude measured 40volts, the same as the battery output for each mode, no variation. The big variation was in the switching frequency which I calculated as 111, 161 and 208Hz.
So it would appear the switching frequency is the main parameter that affects speed. I'll post some 'scope pictures later, maybe tomorrow.
Everything I've read about parameterdesigner (Endless-sphere.com • View topic - Keywin E-bike Lab Parameter Designer Software Manual ver 1.2) is that the three speed limits can be set from 0-120%. 0-100% seems to be a percentage of the free running speed affected by the motor winding and some global limit. 100-120% adds in some advance on the phase angle to provide a kind of turbo boost. I can't find much about default values but I suspect the controller comes with something like 60%-100%-120% Which then corresponds with your 10-15-19mph. Thinking about this, changing the DC and Phase current limit is going to give more or less acceleration and hill climbing ability but not change the speeds. So going to a 20A controller from a 15A controller with everything else the same will give more power, but not more top speed.
Two things I'm still trying to find out.
1) A way of getting the stock values out so there's a base working setting you can get back to. The software at the moment doesn't seem to allow this, so it may involve emails to ecrazyman to find out the stock values.
2) What the 100% speed limit actually means in terms of switching frequency and if that can be increased. Without that I suspect there's no more speed to be had from a given geared hub motor regardless of how much power we feed into it.
For reference, if I lift the rear wheel on the Aurora and apply full throttle I get the same max speeds in each mode as when I'm actually riding. Not too surprising. But again, pointing at a switching frequency limit.