Typical 13.5T RC car inrunners have kv parameters of 3000-3500 depending on the make etc. To spec a direct drive equivalent, multiply the kv of the 13.5T motor by the gear ratio you run and that will get you in the ballpark. For example if you run 25:75 that is a 1:3 ratio, so if your 13.5 motor was a 3000kv then your direct drive motor would need about 1000kv to be roughly RPM-equivalent. That's only part of the equation though, you need to ensure it produces enough torque and can dissipate enough power.
__________________
|