In RC racing, the Final Drive Ratio (FDR) is the ratio of how many times the pinion gear rotates for every rotation of the car's wheels. The higher this number, the more times the pinion rotates per turn of the wheels, producing more torque and acceleration but less top speed. If this number is lower, the wheels turn more closely to the rotation rate of the pinion, producing more top speed, but less torque and acceleration. Looking at it the other way, larger pinions produce a lower FDR, and vice versa.
The Final Drive Ratio is calculated by multiplying the internal/transmission ratio by the secondary ratio, the ratio of the spur's teeth to the pinion:
final drive ratio = internal ratio * ( spur / pinion )
So, let's say you know one of the fast girls at the track has a certain FDR. You can figure out which pinion to use to get that same FDR on your car even if you have a different car by reversing the equation:
pinion = ( internal ratio * spur ) / final drive ratio
For recommendations on a pinion to use for your car, check out our free RC Gear Recommender App.
For help calculating FDR or rollout, try our free RC Gear Ratio Calculator App.