for 1:10 on road 13T brushless motor suggest final drive ratio FDR range is 4.5-5.5
spur gear 86/ motor gear ___ = motor gear ratio ____
diff gear 38T/ 20T = inital gear ration 1.9
final drive ratio FDR = motor gear ration * inital gear ration
therefore you can use motor gear (more accelrate from corner) 30T to 36T (more end speed), depends on the track.