[KenWittlief]

it would be interesting to see what the curves look like with more load on the motors - I think the curves would be far more linear and straighten out.

Controllers usually put out a voltage signal, and that normally corresponds to torque out of the motor (not speed)

so the voltage to speed curve becomes a function of load:

Voltage => torque (into a load) => speed

with no load (or a small load) applied the motor speeds up much easier.

this might also flatten the curve at the upper and lower ends ( the top and bottom 15 levels of the PWM commnand).