How do you calculate gear ratio?

Gear ratio is calculated by dividing the number of teeth on the driven (output) gear by the number of teeth on the driving (input) gear. For example, if the driving gear has 20 teeth and the driven gear has 60 teeth, the gear ratio is 60 ÷ 20 = 3:1. This means the driven gear rotates once for every three rotations of the driving gear.

What is the difference between gear ratio and speed ratio?

Gear ratio and speed ratio are inverses of each other. Gear ratio = Driven teeth ÷ Driving teeth, and it equals the torque multiplier. Speed ratio = Driving teeth ÷ Driven teeth = 1 ÷ Gear ratio. A gear ratio of 3:1 means the output has 3× the torque but 1/3 the speed of the input.

What is a compound gear ratio?

A compound gear train uses multiple gear pairs in series. The overall gear ratio is the product of each individual stage ratio. For example, if Stage 1 has a ratio of 3:1 and Stage 2 has a ratio of 4:1, the compound ratio is 3 × 4 = 12:1. This allows very high ratios in a compact package without needing one very large gear.

How do gear ratios affect torque and speed?

A higher gear ratio (e.g., 4:1) increases output torque by 4× but reduces output speed to 1/4 of the input — this is called a speed reduction or underdrive. A gear ratio less than 1:1 (e.g., 0.7:1) is an overdrive — it increases speed but reduces torque. Ignoring friction losses, the power (torque × speed) remains constant.

How does the final drive (axle) ratio affect my car?

The final drive ratio is the gear reduction in the differential. A higher number (e.g., 4.10) provides more acceleration and towing power but lower top speed and worse fuel economy. A lower number (e.g., 2.73) gives better highway fuel economy and higher top speed but slower acceleration. Common ratios range from 2.73 to 4.56.

Does an idler gear change the gear ratio?

No. An idler gear (placed between the driving and driven gears) reverses the rotation direction of the output without changing the overall gear ratio. The ratio is still determined solely by the driving and driven gears. Idler gears are used when you need the output to rotate in the same direction as the input with spur gears.

How do I calculate RPM from gear ratio?

Output RPM = Input RPM ÷ Gear Ratio. For example, if the input shaft spins at 1,500 RPM and the gear ratio is 3:1, the output shaft spins at 1,500 ÷ 3 = 500 RPM. Conversely, to find the required input RPM: Input RPM = Desired Output RPM × Gear Ratio. Use our RPM mode's reverse-solve toggle to find the gear ratio needed for any target output RPM.

Gear Ratio Calculator

Gear Type

Driving Gear

teeth

Driven Gear

teeth

Compound Gear Train

Basic Gear Ratio

20T driving · 40T driven · Speed Reduction

Gear Ratio

2.00002:1

Gear Details

Ratio, torque, speed, and direction at a glance

Torque multiplier

2.00×

Output torque is amplified

Speed ratio

0.5000

Output ÷ input speed

Direction

reversed

Rotation of output (spur gears: reversed)

Gear type

Speed Reduction

Ratio > 1 — more torque, less speed

What is a Gear Ratio Calculator?

Understand gear ratios and why they matter

A gear ratio calculator determines the relationship between the number of teeth on two meshing gears or sprockets. The gear ratio equals Driven Teeth ÷ Driving Teeth — a ratio of 3:1 means the driving gear must turn three times for each revolution of the driven gear, tripling output torque at the cost of speed. This tool covers simple spur gears, chain/sprocket drives, compound gear trains, automotive drivetrains with tire size and transfer case, and bicycle gearing with custom crank length. Use the reverse-solve mode to find what gear ratio you need for a target speed or output RPM.

Torque & Speed

Calculate torque multiplier and output speed from tooth counts

Vehicle Speed

Find vehicle speed from engine RPM, gear ratios, tire size, and transfer case

Bicycle Gearing

Gear inches, development, gain ratio, and speed at every cadence

How to Use This Gear Ratio Calculator

Step-by-step guide for each calculation mode

Basic Gear Ratio

Select Gear Ratio mode and choose Spur Gears (reverses direction) or Chain/Sprocket (same direction).
Enter teeth on the Driving Gear (input) and Driven Gear (output).
Toggle Compound Gear Train for two-stage setups.
Read the gear ratio, torque multiplier, speed ratio, and output rotation direction.

RPM Calculation

Switch to RPM Calculator mode.
Enter Input RPM and Gear Ratio to see output RPM.
Toggle Solve for Gear Ratio to find the ratio needed for a target output RPM.
The formula with your values is shown below the result.

Vehicle Speed

Switch to Vehicle Speed mode.
Enter Engine RPM, Transmission Ratio, and Final Drive Ratio.
Add a Transfer Case Ratio for 4Lo (e.g. 2.72 for Jeep).
Set your Current Tire Diameter (inches) and optionally enter a New Tire Size like 265/70R17 to see the re-gear impact.
Toggle Solve for Gear Ratio to find the ratio needed for a target speed at your RPM.
Review the Speed at RPM table for speed at common engine RPMs.

Bicycle Gearing

Switch to Bicycle Gearing mode.
Enter Chainring (front) and Rear Cog teeth.
Select or enter the Wheel Size in mm (outer diameter, tire included).
Set Crank Length (default 170mm; common range 165–175mm).
View gear inches, development, gain ratio, and the speed-at-cadence bar chart.

How Gear Ratio is Calculated

The formulas behind every calculation mode

Basic Gear Ratio

Gear Ratio = Driven Teeth ÷ Driving Teeth

Output RPM

Output RPM = Input RPM ÷ Gear Ratio

Vehicle Speed

Speed (mph) = (RPM × π × Tire Dia.) ÷ (Overall Ratio × 1056)

Compound Ratio

Overall = Stage 1 Ratio × Stage 2 Ratio

Reverse Solve

Required Ratio = Input RPM ÷ Target Output RPM

Required Ratio = (RPM × π × Tire Dia.) ÷ (Target Speed × 1056)

Bicycle Gearing

Gear Inches = (Chainring ÷ Cog) × Wheel Dia. (in)

Development (m) = Ratio × Wheel Circumference (m)

Gain Ratio = Development ÷ (2π × Crank Length)

Tire Size Parser

Sidewall (mm) = Width × Aspect Ratio ÷ 100

Tire Diameter (in) = (Sidewall × 2 ÷ 25.4) + Rim Diameter

Example: 265/70R17 → Sidewall = 265 × 0.70 = 185.5 mm → Diameter = (185.5 × 2 ÷ 25.4) + 17 = 31.6″

Real-World Gear Ratio Examples

Common applications across industries

Automotive 1st Gear

A typical first gear ratio of 3.5:1 with a 3.73 final drive gives an overall ratio of 13.06:1 — maximum torque for acceleration from a stop.

Road Bike Climbing

A 34T chainring with a 28T cog gives 1.21:1 — a low gear ratio (about 33 gear inches) ideal for steep climbs.

Industrial Reducer

A two-stage worm gear reducer with 50:1 per stage provides 2500:1 overall ratio — extreme torque for heavy machinery.

Go-Kart Sprocket

An 11T driver with a 60T rear sprocket gives 5.45:1 — balancing top speed with acceleration for karting. Switch to Chain/Sprocket type since direction is maintained.

Tire Size Change

Switching from 265/70R17 (31.6") to 285/70R17 (32.7") changes the effective gear ratio. Use the calculator to see how your speed changes at the same RPM.

Highway RPM Check

With a 0.70:1 overdrive, 3.42 final drive, and 28" tires, 70 mph requires just 2,004 RPM — ideal for fuel-efficient highway cruising.

BMX Racing Gear

A 44T chainring with a 16T cog on a 20" BMX wheel (520mm) gives 55 gear inches — quick acceleration out of the gate for BMX racing.

4Lo Crawl Speed

With a 2.72:1 transfer case in 4Lo, 3.5:1 first gear, 4.10 final drive, and 33" tires, 1,500 RPM yields just 3.8 mph — perfect for rock crawling.

Common Gear Ratio Mistakes

Avoid these frequent errors

Swapping driving and driven gears

The driving gear is the input (connected to the motor/pedals). The driven gear is the output. Swapping them inverts the ratio.

Assuming direction reversal for sprockets

Spur gears reverse rotation direction. Chain, belt, and sprocket drives maintain the same direction. Use the Gear Type toggle to get the correct rotation direction.

Forgetting the final drive or transfer case ratio

For vehicle speed, the overall ratio = transmission ratio × final drive × transfer case ratio (if applicable). Omitting these gives incorrect speed.

Confusing gear ratio with speed ratio

Gear ratio = driven/driving teeth (output torque multiplier). Speed ratio is its inverse (output speed / input speed).

Frequently Asked Questions

Common questions about gear ratios, RPM, vehicle speed, bicycle gearing, and reverse calculations

Vehicle speed = (Engine RPM × π × Tire Diameter) ÷ (Overall Gear Ratio × 1056), where the overall gear ratio = Transmission ratio × Final drive ratio × Transfer case ratio (if applicable), and 1056 converts inches per minute to miles per hour (63,360 inches per mile ÷ 60 minutes per hour). Our Vehicle Speed mode handles this calculation automatically — enter your engine RPM, gear ratios, and tire size.

Gear inches is a measure of bicycle gearing difficulty. It equals the gear ratio multiplied by the overall wheel diameter (rim + tire) in inches. A low number (20–40) means an easy, low gear for climbing. A high number (80–120) means a hard, high gear for speed. For example, a 50T chainring with a 16T cog on a 700c wheel (~680 mm outer diameter) gives about 84 gear inches — a common road cycling gear.

Gain ratio is a more modern measure of bicycle gearing proposed by Sheldon Brown. It equals the development distance (meters per pedal revolution) divided by the crank circle circumference (2π × crank length). Unlike gear inches, gain ratio accounts for crank length and gives a dimensionless number that is directly comparable across different setups. Our calculator uses your actual crank length (default 170mm) for precise gain ratio calculations.

To find the required gear ratio from input and output RPM: Gear Ratio = Input RPM ÷ Output RPM. For example, if your motor runs at 3,600 RPM and you need the output shaft at 600 RPM, the required ratio is 3,600 ÷ 600 = 6:1. Our RPM Calculator includes a "Solve for Gear Ratio" toggle — switch it on, enter your input and target output RPM, and the required ratio appears instantly with the formula shown.

To find the overall gear ratio needed for a specific speed: Overall Ratio = (Engine RPM × π × Tire Diameter) ÷ (Target Speed × 1056). For example, to cruise at 70 mph at 2,500 RPM with 28" tires: Ratio = (2,500 × π × 28) ÷ (70 × 1056) = 219,911 ÷ 73,920 = 2.97:1. Use our Vehicle Speed mode's reverse-solve toggle to calculate this automatically — enter your engine RPM, target speed, and tire size, and the required overall ratio appears with the full formula.

Larger tires reduce the effective gear ratio — your engine turns slower at the same speed. To compensate and maintain the same RPM-to-speed relationship, multiply your current ratio by (new tire diameter ÷ old tire diameter). For example, switching from 28" to 33" tires: compensation factor = 33 ÷ 28 = 1.18×. If your current final drive is 3.73, the equivalent would be 3.73 × 1.18 = 4.40 to maintain the same highway RPM. Use our calculator's tire size parser (enter "265/70R17") to quickly get the exact diameter and compare speeds.

Go-karts use chain and sprocket drives which maintain the same rotation direction (unlike spur gears). The ratio = Rear Sprocket Teeth ÷ Clutch/Driver Sprocket Teeth. For example, a 60T rear sprocket with an 11T driver gives 60 ÷ 11 = 5.45:1 — common for adult karts balancing acceleration and top speed. For junior karts, higher ratios like 6:1 or 7:1 provide more low-end torque. Use our Basic mode with the Chain/Sprocket gear type selected.

For climbing steep hills, aim for gear inches between 25–35 (e.g., 34T chainring ÷ 32T cog on a 700c wheel = ~30 gear inches). For flat-road speed, 80–100 gear inches is typical (e.g., 50T ÷ 14T = ~95 gear inches). For mixed terrain, a compact crankset (50/34T) with an 11-32T cassette covers a wide range. Our calculator shows gear inches instantly and the speed-at-cadence chart helps visualize how fast you'll go at different pedaling rates.

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Last updated Jun 13, 2026

Gear Ratio Calculator

Gear Details

What is a Gear Ratio Calculator?

How to Use This Gear Ratio Calculator

How Gear Ratio is Calculated

Real-World Gear Ratio Examples

Common Gear Ratio Mistakes

Frequently Asked Questions

Embed Gear Ratio Calculator

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