Final drive ratio (FDR) is how many turns the motor makes for one turn of the wheels on an RC car, calculated as the internal transmission ratio multiplied by spur teeth divided by pinion teeth.
FDR = internal transmission ratio × (spur teeth ÷ pinion teeth). Team Associated’s own calculator writes it as (spur ÷ pinion) × trans ratio — same math. The internal ratio is fixed by your chassis: a Team Associated RC10B6.4 is 2.60:1, so a 78-tooth spur on a 24-tooth pinion gives (78 ÷ 24) × 2.60 = 8.45. Higher FDR means more acceleration and less top speed; lower FDR means the opposite. Most racers tune with the pinion because it’s a two-minute swap, and treat the spur as the coarse adjustment. FDR ignores tires entirely, which is fine for comparing two setups on the same car and useless the moment tire diameter changes.
Get the FDR wrong and you're either bogging out of every corner or feeding the motor more load than it can shed as heat.