21T pinion, 46T spur
A 21T pinion on a 46T spur is a 2.190:1 spur ratio. With an assumed 2.85:1 internal ratio that is a 6.24:1 final drive, which on a 2050KV motor at 4S with 6.3 in tires works out to about 91.1 mph theoretical and 77.4 mph realistic.
ShareGoo Mod 1 Hardened Steel M1 Pinion Gear Sets 19T 20T 21T
- 1/8
Pinion sets in Mod 1, the pitch this 46T spur uses. Buying a set lets you step one tooth at a time instead of guessing.
$12.99 listed price, changes often
Check price on AmazonWhat final drive does 21/46 give?
The spur ratio is 46 / 21 = 2.190. Your car's transmission or differentials multiply it. The internal ratio varies by model, so here is the final drive across the common range; find yours in the manual next to the gearing chart.
| Internal ratio | Final drive (FDR) | Character |
|---|---|---|
| 2:1 | 4.38:1 | tall: speed, more heat |
| 2.5:1 | 5.48:1 | tall: speed, more heat |
| 2.72:1 | 5.96:1 | tall: speed, more heat |
| 3:1 | 6.57:1 | tall: speed, more heat |
| 3.5:1 | 7.67:1 | tall: speed, more heat |
How fast is 21/46?
Example: a 1/8 6S monster truck, 2050KV motor, 6.3 in tires and an assumed 2.85:1 internal ratio (FDR 6.24:1). Theoretical speed is a no-load ceiling; the realistic figure assumes about 85 percent of it, which is typical on stock tires. For your own motor and tires, use the gear ratio and top speed calculator.
| Battery | Motor RPM | Theoretical | Realistic |
|---|---|---|---|
| 4S LiPo (14.8 V) | 30,340 | 91.1 mph | 77.4 mph |
| 6S LiPo (22.2 V) | 45,510 | 136.6 mph | 116.1 mph |
What does one tooth up or down do?
Speed scales with the pinion on a fixed spur. The heat rating compares each option with 21T, assuming 21T runs at a sensible temperature today.
| Swap | Speed change | Motor heat |
|---|---|---|
| 21T to 18T | -14.3% | cooler |
| 21T to 19T | -9.5% | cooler |
| 21T to 20T | -4.8% | cooler |
| 21T to 22T | +4.8% | low |
Which cars use a 46T Mod 1 spur?
The 46T spur on this page is an Arrma Mod 1 spur for the 29 mm center diff used on Arrma 1/8 6S trucks. Pitch matters more than tooth count: Mod 1 gears only mesh with Mod 1 gears. Set the mesh with a strip of paper between the gears as you tighten the motor, then remove it; there should be a tiny amount of play. See how to change a pinion gear.
21T on other spurs
Every pinion on the 46T spur
- 13T / 46T: 3.54:1
- 14T / 46T: 3.29:1
- 15T / 46T: 3.07:1
- 16T / 46T: 2.88:1
- 17T / 46T: 2.71:1
- 18T / 46T: 2.56:1
- 19T / 46T: 2.42:1
- 20T / 46T: 2.30:1
- 22T / 46T: 2.09:1
Gears and a thermometer for this swap
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Frequently asked questions
What is the gear ratio of 21/46?
46 divided by 21 is 2.190, so the spur ratio is 2.19:1. The final drive ratio multiplies that by your car's internal transmission or differential ratio, which is printed in the manual. At a 2.72:1 internal ratio it is 5.96:1; at 3:1 it is 6.57:1. Use the calculator with your own internal ratio for an exact figure.
Is 21T faster than 20T on a 46T spur?
Yes. On the same spur, speed scales with pinion teeth, so 21T is about 5.0 percent faster than 20T at the same motor RPM. The motor also works harder at every speed, so it runs warmer. Check the temperature after two or three minutes of running before going up another tooth.
Will a 21T Mod 1 pinion fit my car?
Only if the spur is also Mod 1 and the motor mount has enough adjustment for the pinion size. Pitch must match exactly; Mod 1 gears will not mesh correctly with other pitches. Bore size must match the motor shaft, commonly 3.175 mm (1/8 in) on 1/10 motors and 5 mm on larger motors. The spur here is an Arrma Mod 1 spur for the 29 mm center diff used on Arrma 1/8 6S trucks.
RCCarCalc is researched guidance, not professional advice. Figures come from published manufacturer specifications, listings and verified owner reports, not hands-on testing. Speeds are theoretical or as listed; real results depend on gearing, battery, tires and terrain. See how we research.