sebi9 wrote:Other question : if I put more slowly the printing speed, do I need to put the extrusion speed lower?
No. The actual extrusion speed is always calculated by the firmware, based on the printing speed, the filament diameter, the layer height, and the trace width (and the Extrusion Multiplier). When you specify an Extrusion Speed in the EEPROM, that's really just a Maximum Allowed speed.
The firmware always figures out which speed is the limiting one (if a Maximum Allowed is reached) and then calculates the other speeds to conform.
If you suspect under-extrusion or over-extrusion, you should check your Extruder Calibration (steps per mm) and the Filament Diameter that you are using in your slicer (it's best to measure the average filament diameter with digital calipers). Then, if necessary, adjust the Extrusion Multiplier.