How to Calculate RC Car Gear Ratio
Final drive ratio (FDR) equals (spur teeth divided by pinion teeth) multiplied by the internal transmission or diff ratio. A 17-tooth pinion with an 87-tooth spur and a 2.6:1 internal ratio gives an FDR of about 13.3:1.
Gear ratio in an RC car is the number of times the motor shaft must turn to spin the wheels once, found by combining the spur-to-pinion ratio with the vehicle's internal gearbox ratio.
5Pack ShareGoo Steel 48P 3.175mm Pinion Gear Sets 13T 14T 15T
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The 5Pack ShareGoo Steel 48P 3.175mm Pinion Gear Set, 13T-17T lets you test a full range of ratios against one spur without buying five separate gears. Start with the middle tooth count and work outward once you know whether you want more torque or more top speed.
$11.49 listed price, changes often
Check price on AmazonWhat is gear ratio in an RC car?
Every RC car has at least two gear reductions between the motor and the wheels. The first is external and adjustable: the pinion gear on the motor shaft meshes with the spur gear on the transmission. The second is internal and usually fixed by the chassis design: gears inside the transmission or differential housing that further reduce RPM before it reaches the wheels.
Gear ratio, or final drive ratio, is the combination of both reductions expressed as a single number, like "13.3:1." That number tells you how many motor revolutions it takes to turn the wheels once. A bigger number means more torque and less top speed for a given motor and voltage; a smaller number means the opposite.
How to calculate gear ratio for an RC car, step by step
Step one: find your spur-to-pinion ratio. Divide the spur gear's tooth count by the pinion gear's tooth count. An 87-tooth spur with a 17-tooth pinion gives 87 / 17 = 5.12.
Step two: find your internal ratio. This is set by the transmission or differential gearing inside the chassis and is usually published in the manual or the manufacturer's spec sheet as a single number, commonly somewhere around 2.0:1 to 3.0:1 on typical 2WD short course trucks and buggies.
Step three: multiply the two. Final drive ratio = spur-to-pinion ratio x internal ratio. Using 5.12 and an internal ratio of 2.6:1: FDR = 5.12 x 2.6 = 13.31:1.
| Step | What you need | Example |
|---|---|---|
| 1 | Spur teeth / pinion teeth | 87T / 17T = 5.12 |
| 2 | Internal (transmission/diff) ratio | 2.6:1 |
| 3 | Final drive ratio = step 1 x step 2 | 5.12 x 2.6 = 13.31:1 |
How do I turn final drive ratio into wheel speed?
Once you have FDR, wheel RPM is just motor RPM divided by FDR. Motor RPM itself comes from KV multiplied by voltage; see RC motor KV explained if that part is unfamiliar.
From wheel RPM you can get theoretical top speed with: mph = wheel RPM x tire diameter (inches) x pi x 60 / 63,360. Real-world speed usually lands at 80 to 90 percent of that theoretical number because of rolling resistance, weight, and voltage sag under load.
What changes if I swap the pinion?
Swapping only the pinion is the fastest way to change gear ratio without touching the internal ratio, which is why it is the most common hop-up move. A smaller pinion raises the spur-to-pinion ratio (more reduction), giving more torque and acceleration at the cost of top speed and slightly more motor RPM needed to reach the same wheel speed.
A larger pinion lowers the ratio, trading torque for a higher theoretical top speed while loading the motor harder per revolution. See RC car gearing explained for the full tradeoff table and how to change a pinion gear for the swap itself.
Full example: pinion, spur, KV, and top speed together
- Pinion: 17 teeth. Spur: 87 teeth. Spur / pinion = 87 / 17 = 5.12.
- Internal ratio (typical for this class of chassis): 2.6:1. Final drive ratio = 5.12 x 2.6 = 13.31:1.
- Motor: 3500KV on a 2S (7.4V) pack. Motor RPM = 3500 x 7.4 = 25,900 RPM.
- Wheel RPM = motor RPM / FDR = 25,900 / 13.31 = 1,946 RPM.
- Assume a 4.4-inch tire diameter. Theoretical mph = 1,946 x 4.4 x pi x 60 / 63,360 = about 25.5 mph.
- Real-world speed, at 80 to 90 percent of theoretical, lands around 20 to 23 mph, which matches what stock 2S brushless short course trucks typically run.
Gear for this job
Higher-tooth pinion set for more top speed
48P 19T 20T 21T 22T 23T Pinion Gear with Screw Driver
The 48P 19T-23T Pinion Gear Set for 3.175mm Shaft lowers your ratio for more theoretical top speed. Only step up into this range once you have confirmed motor temperature is under control at the lower end; a bigger pinion loads the motor harder, not less.
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Steel spur gear to build your own ratio table against
BRKRC Steel Spur Gear 48p 87T Gears for Axial SCX10 1/10 RC
The BRKRC Steel Spur Gear 48P 87T is a common reference spur size. Pair it with the pinion sets above and run each combination through the gear ratio and top speed calculator before you buy anything else.
$9.99
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Frequently asked questions
How do I calculate gear ratio for my RC car?
Divide your spur gear's tooth count by your pinion gear's tooth count, then multiply that by your vehicle's internal transmission or diff ratio. The result is your final drive ratio. For example, an 87-tooth spur with a 17-tooth pinion and a 2.6:1 internal ratio gives 87/17 x 2.6, or about 13.3:1.
What is the difference between spur-to-pinion ratio and final drive ratio?
Spur-to-pinion ratio is only the external reduction between the motor's pinion gear and the transmission's spur gear. Final drive ratio also includes the internal gearbox or differential reduction inside the chassis, which is usually fixed unless you install different internal gears. FDR is the number that actually predicts wheel speed.
Do I need to know my internal gear ratio to calculate FDR?
Yes. Without it you only have the spur-to-pinion ratio, which is not the same thing as final drive ratio. Manufacturers usually publish the internal ratio in the vehicle's manual or spec sheet; if you cannot find it, a rough estimate around 2.0:1 to 3.0:1 is typical for most 2WD short course trucks and buggies.
How does gear ratio affect top speed and acceleration?
A lower final drive ratio (achieved with a larger pinion or fewer internal reduction stages) raises theoretical top speed but reduces acceleration and torque, and increases load on the motor per revolution. A higher final drive ratio does the opposite: stronger acceleration, lower top speed, generally cooler motor operation for the same throttle input.
Can I use this same gear ratio formula for a crawler?
Yes, the formula is identical; only the target changes. Crawlers intentionally run very high final drive ratios, often well above 20:1, because the goal is low-speed torque and control, not top speed. See RC crawler overdrive and underdrive for how crawlers push this further.
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.