Yeah, well, they're pretty linear, but not perfectly, and that's true, my error could be anywhere in the system, as i'm using an SS -> G540 -> KBMM-125 (motor speed control) -> motor.
I can control motor rpm within mach 3 with the S word, but the motor rpm is never what I actually want, it's correct at around 1800 rpm, but then at s3000 actual rpm is like 6k and at s1000 actual is like 340 so there seems to be a pretty epic curve. I'm using an ac/dc motor with a max rpm of 30k, i believe it was removed from a vacuum cleaner it doesn't have permanent magnets, but rather a coil.