This page documents all the motors that I know of that provide good to excellent performance in the X400. I have also included a recommended pinion to use. I will add to this table as new motors are tested.

If you have a motor not on this list but would like to calculate (roughly) the required pinion it can be done as follows :

target rpm * teeth on main gear / motor kv / voltage / motor efficiency

In the case of the X400 this equates to an equation for the AON 2815-3000 with 3S lipo as follows :

2400 * 138 / 3000 / 11.1 / 80% = 12T or 13T (the result was 12.4)