Traxxas Mod 0.8 TRX-4 Spur Gear (45T)

You will need this 45t spur in order to reduce the pinion size between 16t to 9t for your TRX-4 Sport. The gearing ratio can be reduced to between 44.44 and 79.00 depending on the pinon size chosen (stock ratio is 38.51). The stock spur of 39t only allows the smallest pinon to be 15t which is a ratio of 41.08 so if you need to go lower you will need this 45t spur to make the gears mesh properly. This spur gear is plastic and is compatible with any 32p (pitch) or Mod 0.8 gearing within the mentioned tooth (t) number limits. You will normally need a slipper clutch rebuild kit to install the clutch friction material onto this spur gear on both sides as the spur gear does not come with it. However if your careful you can remove the old clutch material using a hobby blade and using CA glue to install on the new spur provided it's not too worn. Note the 15t and the 16t pinion will fit with either the stock or this spur gear. This also gives you the most tuning range using both spur gears. My personal opinion is the 15t pinion worked for me using the stock 21t Titan motor on 3S on a stock TRX-4 Sport. Yours will vary on driving personality, motor, ESC, battery and drivetrain setup.

If you put an axe system in a trx4 sport, You will need this too. Sport comes with a smaller spur that can't mesh with an 11t pinion. Follow gear mesh charts and use traxxas aluminum motor mounting plates as well. Set it and forget it!