Out of sync dual extruder

I have an A20M dual extruder printer of Geeetech. It has 2 similar extruder stepper motors which feed 1 mixing nozzle via seperate bowden tubes.


I'm bussy re-calibrating the printer. And ofcourse began with calibrating the extruder steppers.


But I've found out that E1 feeds around 1mm more (without nozzle connected) then E0 does (feeding 100mm). I can't seperately configure the steps/mm because Marlin 2.0.x seems to refuse this when MIXING_EXTRUDER is configured.
It only shows me 1 E-stepper setting. And it uses the same value for E0 and E1.


Idler spring tensions seems to be equal, so guess this is not the problem.


I also changed the cables from one to another and tested again. Which gave me the same results on the same motors. So it's also nothing with electronics or stepper-driver settings/resistance.


So I guess it's some diffrence in tolerance of the motors itself? Is there a way to compensate the diffrences?

Got an small update in the meanwhile:

- So I've switched the cables on the motors with each other, does not seem to make anny diffrence. So I asume the stepper-driver settings are equal, otherwise results should have been switched too I suppose.
- I've measured the coils on both of the stepper motors. results were equal.
- I had 2 diffrent colors (so probably 2 diffrent types too) of filament, white on E0 and yellow on E1. When I switched the filament rolls on the extruder, problem seemed reversed. So I thought maybe it's something about filament density.
- I took a new spool of filament and tested it on both of the extruders. Then the problem and difference was similar again, E1 feeding slightly more then E0.

So looks like I'm back to the start again...

The error of 1 percent is really unlikely to be a problem. ,I doubt extruder gears are less than 1 percent different in size.