Final drive ratio calculator
Work out FDR from your spur, pinion and internal ratio.
How it works
Final drive ratio is how many turns of the motor it takes to turn the wheels once:
FDR = (spur ÷ pinion) × internal ratio
The spur and pinion are the gears you swap at the track. The internal ratio is fixed by the transmission and comes from the manual, so it is the one number you cannot read off the car. A higher final drive ratio gears the car down: more acceleration, lower top speed, and less heat in the motor. A lower one does the opposite.
An 84 tooth spur on a 28 tooth pinion is a 3.00:1 gear ratio, and through a 2.6 internal ratio that is a 7.80:1 final drive — the motor turns 7.8 times for every turn of the wheels.
Spur and pinion combinations
Gear ratios before the internal ratio is applied. Multiply by your transmission’s internal ratio to get the final drive.
| Spur | 24T pinion | 26T pinion | 28T pinion | 30T pinion |
|---|---|---|---|---|
| 78T | 3.25:1 | 3:1 | 2.79:1 | 2.6:1 |
| 81T | 3.38:1 | 3.12:1 | 2.89:1 | 2.7:1 |
| 84T | 3.5:1 | 3.23:1 | 3:1 | 2.8:1 |
| 87T | 3.63:1 | 3.35:1 | 3.11:1 | 2.9:1 |
| 90T | 3.75:1 | 3.46:1 | 3.21:1 | 3:1 |