Rollout is the distance an RC car travels along the ground for one full revolution of the motor, calculated as tire circumference divided by final drive ratio.
Measure your tire’s diameter, multiply by pi for circumference, then divide by your final drive ratio: rollout = (tire diameter × π) ÷ FDR. A 1/10 buggy with a 78-tooth spur, a 24-tooth pinion and a 2.60:1 internal transmission ratio runs an FDR of 8.45; on an 86mm rear tire that’s (86 × 3.1416) ÷ 8.45 ≈ 32mm of rollout. Less rollout means more punch and less top speed; more rollout is the reverse. Racers prefer rollout over raw FDR because it follows tire size — foam tires shrink through a run and sanded rubber does the same, so your gearing effectively changes without you touching a gear.
Gear to the wrong rollout and you either bog off the corner or cook the motor chasing top speed you never use.