The spur gear is the large gear an RC car's pinion drives, mounted to the transmission — sometimes on a slipper clutch or, on a 4WD car, a differential. Its tooth count sets final drive ratio with the pinion.
More spur teeth raises FDR (more punch); fewer lowers it (more top speed). Most racers leave the spur alone and gear with the pinion, because a spur change usually means pulling the gear cover and often the slipper. Kits ship options for exactly that reason — the RC10B6.4 comes with 78T and 72T 48-pitch spurs, the 72T listed specifically for stock-motor use. The spur must share the pinion’s pitch, and because it’s the biggest, softest gear in the mesh it’s usually the first part to show damage: check it for chipped or missing teeth whenever the drivetrain starts making noise. Reset the mesh after any spur or pinion change.
The spur is your coarse gearing step and your drivetrain's early-warning light, so a chipped tooth you didn't catch in the pits becomes a DNF in the main.