This is the Associated 75 Tooth 48 Pitch Spur Gear for the RC10B4 and RC10T4.
Associated spur gears are a smaller design for gear ratios used with 17.5, 13.5 and 10.5 brushless stock motors. This spur gear moves motor forward for better balance and more predictable handling.
One 48 Pitch 75 Tooth Spur Gear
Installation on the vehicle
Outer Diameter: 40.7mm (1.6")
Inner Diameter: 4.75mm (0.19")
Width: 5.25mm (0.21")