See: Team Associated 

Team Associated have introduced a set of underdrive differential gears for the RC8B3.1 and RC8B3.1e.This gear set for V3 differentials is great for low traction tracks. The underdrive gear set slows down the rear tyre rotation compared to the front, making it more stable on low grip surfaces. The set comes including 42T ring gear and 12T pinion gear.