The Traxxas 2-Speed Gear Set (#TRA5585) is a stock replacement transmission gear set built for the Traxxas Nitro Jato (2.5 and 3.3 models). 
It pairs a 17-tooth steel input gear with a 29-tooth nylon second-speed gear.
Measures roughly 1/2" (12 mm) in height and 1-1/4" (30 mm) in diameter.