Glockcnc 1200w motor upgrade

Hey folks, I bought 2 Glockcnc motor upgrades (1200w) for my taig mill and lathe and it's driver is giving me problems. I can't tune and match the motor speed to my encoder readings with enough consistency across the scale. I emailed Jim at Glockcnc two weeks ago but he couldn't help me. So I'm hoping someone else has the same motors, maybe someone found a solution?

Here is the problem:

The cnc controller (acorn) speed control voltages (0-10v) values have been tested and are accurate

Motor maximum speed is rated at 5400 rpm.
I set the driver max speed to 5400 rpm

the pulley diameter on the motors is 3.116, the pulley diameter on the spindle (with encoder) is 1.816, the gear ratio is 1.715

5400 rpm of max speed on motor gives me a 9261 rpm max speed in Acorn
So I set max spindle speed in Acorn to 9261, minimum speed to 0 rpm,

I'm trying to match the commanded motor speed to the spindle encoder reading:

run the test for different rpms:

1k=840 rpm
3k=2872
5k=4940
7k=7002
8k=8065


This shows me that these settings work well for 7k rpm, at 7k rpm the encoder matches the motor.
below 7k the encoder value is below the commanded values, and the high range goes above

Lets try to tune this a bit, so my 5k is better tuned:
Change acorn maximum speed from 9261 rpm to 9000 rpm:

1k=870
3k=2968
5k=5096
7k=7230
8k=8308

the 5k is closer now, below 5k the values are under, above 5k the values are above.
I need to get accurate rpms at 640 rpm so I can do rigid taping. If I lower the max speed I can get very close, but that lowers my maximum speed a lot and all values above 640rpm are exponentially higer, 8k becomes almost 10k



Does this make sense? Does it show that I’m missing a bottom scale parameter somewhere hidden in the driver?
The manual is very short and very badly translated and barely makes sense. I looked and tried every parameter, I did not find a setting that would act as a bias or gain.

Anybody experienced this?