Assume somebody has a Monoprice Maker Select and has changed from the original brass extruder gear to a D4 Plus one, which is slightly smaller (10.6 vs 10.9 mm outer diameter).
This person might be wondering if he/she needs to compensate for this difference, and which setting that would be in Cura IIIP.
(Have never calibrated the extruder and am still not sure it is absolutely necessary, since the difference in the gear size is only 3%.)
If you change the extruder wheel for a different sized wheel, you need to calibrate the extruder to make sure that if you ask to extrude 100 mm it actually extrudes 100 mm. This answer on the question "How do I calibrate the extruder of my printer?" describes how to do that.
It is not required to flash your firmware. The G-code command M92 can be used to set the new amount of steps for the extruder. The Monoprice Maker Select has a Melzi control board that is running Repetier firmware. This G-code command is supported by Repetier firmware.
You need to be able to connect a so-called terminal to your printer. Applications as Repetier-Host, Pronterface, OctoPrint, and probably many more have so-called terminals where you can interface with the printer by sending command to it when the printer is connected through USB (please mind the communication speed of the board, called Baud rate, these are not the same for all boards).
Sending M503 will report the current settings for M92, e.g.:
M92 X100.00 Y100.00 Z400.00 E100.00
Extrude 100 mm without the hotend attached so you can measure the amount that is extruded. If that is 80 mm you need more steps $ \frac{100\ mm}{80\ mm} \times 100\ steps/mm = 125\ steps/mm $
You now need to send M92 E125 and the new steps are set. Use M500 to store the setting.
You could also change the flow extrusion parameter in your slicer, but it generally not good practice, it is better to fix the printer rather than adjusting in the slicer. However, if you do want to fix it in the slicer, as mentioned in the comments, you can also add steps setting in the start G-code script: "To get around flashing the new values to the ROM, you can add this to the machine settings in Cura under "Start Gcode" this way it will append your values at the start of every print.". Note that other slicers have similar functionality.
This will affect your extrusion rate. The best place to change this would be in your E steps located in the firmware but you can also compensate for this by changing the flow percentage in Cura, a setting of 103% (10.9/10.6) should get you close.
