Calculate gear ratio using three different methods: teeth count, input/output speed, or compound gear trains.
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Every gear set in a vehicle’s drivetrain serves a purpose, multiplying torque, reducing speed, or both. Whether you’re building a transmission, setting up a rear axle, or designing a custom gear train, understanding the gear ratio is essential. The gear ratio determines how many times the input shaft turns for each revolution of the output shaft, and it directly affects the vehicle’s acceleration, top speed, and the torque available at the wheels.
This gear ratio calculator gives you three distinct methods to find the ratio, matching the data you already have. Choose the teeth count method when you know the number of teeth on your gears. Use the speed method when you have input and output RPM readings. Select the compound gear train method for multi-stage gear sets. The tool handles the math and delivers results in one view. Whether you’re an automotive enthusiast, a mechanical engineer, or a student learning about powertrains, this calculator delivers accurate answers in seconds. All calculations run locally in your browser, keeping your data private.
Choose your unit system: Imperial (RPM, lb-ft) or Metric (RPM, Nm) from the dropdown at the top.
Select your calculation method from the three options: Number of Teeth, Input/Output Speed, or Compound Gear Train.
For Teeth method: enter the driving gear teeth count, driven gear teeth count, input RPM, and input torque.
For Speed method: provide the input speed, output speed, and input torque.
For Compound method: enter the teeth counts for all four gears in a two-stage train, plus input RPM and torque.
Review your gear ratio, output speed, output torque, speed reduction percentage, and torque multiplication.
The calculator applies three different formulas depending on which method you select, each derived from the fundamental relationships between gear teeth, speed, and torque.
Method 1: Number of Teeth:
Formula: Gear Ratio = Driven Teeth / Driving Teeth
This is the most straightforward method. The ratio is simply the number of teeth on the driven gear divided by the number of teeth on the driving gear. A ratio greater than 1 means speed reduction and torque multiplication.
Method 2: Input/Output Speed:
Formula: Gear Ratio = Input Speed / Output Speed
When you know the rotational speeds of the input and output shafts, the ratio is the input speed divided by the output speed. This method is useful when you have RPM readings from a tachometer or data logger.
Method 3: Compound Gear Train:
Formula: Total Ratio = (G2 / G1) × (G4 / G3)
For a two-stage compound gear train, the total ratio is the product of the individual stage ratios. The first stage is gear 1 (driving) to gear 2 (driven), and the second stage is gear 3 (driving) to gear 4 (driven).
Formula: Output Speed = Input Speed / Gear Ratio
Formula: Output Torque = Input Torque × Gear Ratio
Speed and torque are inversely related through the gear ratio — a higher ratio means lower output speed but higher output torque. This relationship is the foundation of gearbox design.
Consider a gear set with a 20-tooth driving gear and a 60-tooth driven gear. The input speed is 3,000 RPM and the input torque is 100 lb-ft. Let’s calculate the gear ratio, output speed, and output torque.
Step 1: Identify the known values
Driving Gear Teeth = 20
Driven Gear Teeth = 60
Input RPM = 3,000
Input Torque = 100 lb-ft
Step 2: Calculate the gear ratio
Ratio = 60 / 20 = 3.0 : 1
Step 3: Calculate the output speed
Output RPM = 3,000 / 3.0 = 1,000 RPM
Step 4: Calculate the output torque
Output Torque = 100 × 3.0 = 300 lb-ft
Step 5: Calculate the speed reduction
Reduction = (1 – 1/3.0) × 100 = 66.7%
Step 6: Calculate the torque multiplication
Multiplication = 3.0 ×
Interpretation: This gear set has a 3:1 ratio, which means the output shaft turns once for every three turns of the input shaft. The speed is reduced by 66.7%, while the torque is multiplied by 3 times. This is a classic setup for a first gear in a transmission or a differential.
Provides three distinct calculation methods to match any data you have available.
Supports both Imperial (RPM, lb-ft) and Metric (RPM, Nm) unit systems.
Displays gear ratio, output speed, output torque, speed reduction, and torque multiplication.
Helps vehicle builders and tuners select the right gearing for their applications.
Perfect for automotive enthusiasts, students, mechanics, and engineers.
Updates results instantly as you change inputs, perfect for exploring what-if scenarios.
Runs entirely client-side with no server communication, keeping your data private.
Free to use on any device with responsive design.
Gear ratio is the relationship between the rotational speeds of two meshing gears. It’s expressed as a ratio, such as 3:1, meaning the input gear turns three times for each revolution of the output gear.
The formula is: Gear Ratio = Driven Teeth / Driving Teeth. For example, a 20-tooth driving gear and a 60-tooth driven gear gives a 3:1 ratio.
A compound gear train is a system with two or more pairs of gears on common shafts. The total gear ratio is the product of the individual stage ratios. For example, a 2:1 stage followed by a 3:1 stage gives a 6:1 total ratio.
Speed and torque are inversely related through the gear ratio. A higher ratio (e.g., 3:1) reduces output speed and increases output torque. A lower ratio (e.g., 0.7:1) increases output speed and reduces output torque.
Gear ratio typically refers to a single gear pair. Final drive ratio is the overall ratio from the engine to the wheels, which includes the transmission gear ratio and the axle ratio. The final drive ratio determines the vehicle’s overall performance.
For a typical street car, a final drive ratio around 3.0–3.5:1 balances acceleration and highway cruising. Performance cars often use 3.5–4.0:1 for stronger acceleration.
Overdrive is a gear ratio less than 1:1, meaning the output shaft turns faster than the input shaft. This reduces engine RPM at highway speeds, improving fuel economy and reducing engine wear.
Larger tires effectively lower the gear ratio because they cover more distance per revolution. This reduces engine RPM at a given speed and lowers torque at the wheels. The calculator doesn’t directly account for tire size, but the relationship is important when selecting gears.
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