"传动小齿轮" can be translated as "drive pinion" or "transmission pinion" in English.
"Drive pinion" is often used in the context of machinery and vehicles where the pinion is the driving gear that meshes with a larger gear (such as a ring gear in a differential) to transmit power.
"Transmission pinion" specifically emphasizes its role in the transmission system of a vehicle or machinery, where it helps in changing the speed or torque of the output.
Both terms are acceptable, and the choice between them may depend on the specific context or industry terminology.