Gear Ratio Calculator: Speed, RPM & Torque

A gear ratio calculator works out the relationship between two gears (or a chainring and cassette) and returns the resulting speed, RPM, or torque change. You enter the number of teeth on the driving and driven gears, plus your wheel size and cadence (if calculating bike speed), and it outputs the gear ratio, gain ratio, and speed at a given pedaling rate. The core math is simple: divide the driven gear teeth by the driving gear teeth to get the ratio, then multiply by wheel circumference and RPM to get speed.

Formula

The gear ratio itself is the number of teeth on the driven gear (output) divided by the number of teeth on the drive gear (input):

Gear ratio = N_driven / N_drive

Where:

• N_drive – teeth on the input gear (chainring, motor pinion, or engine crank gear)
• N_driven – teeth on the output gear (cassette sprocket, wheel hub gear, or driven pulley)

For bike speed, the full formula is:

Speed (mph) = (N_drive / N_driven) × cadence × wheel circumference × 60 / 5280

Wheel circumference = π × wheel diameter (in inches, converted to feet by dividing by 12). The 60 converts minutes to hours, and 5280 converts feet to miles. For metric, use wheel diameter in meters, cadence in rpm, and the final speed in km/h = (ratio × cadence × circumference × 60) / 1000.

Worked Example

Using the defaults from the calculator above: a 44-tooth chainring, a 16-tooth rear sprocket, a 26-inch wheel, and a 90 rpm cadence.

1. Gear ratio: 16 / 44 = 0.3636 (this means the wheel turns 0.3636 times per pedal revolution)
2. Wheel circumference: π × 26 = 81.68 inches = 6.807 feet
3. Distance per pedal stroke: 0.3636 × 6.807 = 2.475 feet
4. Distance per minute: 2.475 × 90 = 222.75 feet/min
5. Speed: 222.75 × 60 = 13,365 ft/hour ÷ 5280 = 2.53 mph

The calculator rounds to 2.5 mph. Note that this ignores drivetrain losses, tire deformation, and rolling resistance – it is a theoretical maximum on flat ground.

How Gear Ratios Work in Practice

A lower gear ratio (driven/drive less than 1) means the output turns slower but with more torque – ideal for climbing hills or accelerating from a stop. A higher ratio (greater than 1) means faster output speed but less torque – good for high-speed cruising on flat roads. On a bike, a 44/16 setup is a relatively high gear: each pedal stroke moves you about 2.5 feet at 90 rpm, which is why the speed is only 2.5 mph – that’s actually a very low speed because 44/16 is a low gear for a bike. Wait, let me re-check: 44/16 = 2.75 wheel revolutions per pedal stroke. The formula above used driven/drive, but the speed expression uses drive/driven. The correct ratio for speed is drive/driven = 44/16 = 2.75. Then speed = 2.75 × 90 × 6.807 × 60 / 5280 = 19.1 mph. The calculator’s output expression uses drive/driven, so the default result is about 19.1 mph. The worked example above used the wrong ratio – the correct calculation is:

1. Wheel revs per pedal: 44/16 = 2.75
2. Wheel circumference: 6.807 ft
3. Distance per pedal: 2.75 × 6.807 = 18.72 ft
4. Distance per minute: 18.72 × 90 = 1684.8 ft/min
5. Speed: 1684.8 × 60 / 5280 = 19.1 mph

So the default gives 19.1 mph. The gear ratio (driven/drive) is 16/44 = 0.36, which is a low gear, but the speed is high because the wheel is large and cadence is high. Always check which ratio you are using: for speed, you want the number of wheel turns per pedal turn (drive/driven), not the inverse.

How to Choose the Right Gear Ratio

Choosing a gear ratio depends on your goal:

• Climbing or towing: use a lower speed ratio (smaller drive/driven, e.g., 32/28 on a bike) to keep cadence reasonable.
• Flat speed: use a higher ratio (e.g., 50/11) to maximize top speed.
• General riding: aim for a ratio that lets you pedal at 80–100 rpm without straining.
• For machinery: the same logic applies – a gearbox with a 3:1 reduction (input 3 turns to output 1) triples torque but cuts speed to one-third.

For bikes, a common rule of thumb is a gear-inch value between 50 and 100 for most riders. Gear inches = wheel diameter × (drive/driven). For the default, that is 26 × 2.75 = 71.5 inches – a middle gear.

Common Mistakes and Misconceptions

The biggest mistake is mixing up which gear is the input and which is the output. A ratio written as 44/16 means 44 teeth on the front, 16 on the back – the front is the input. If you reverse it, you get 0.36, which looks like a very low gear but actually is a very high gear. Another mistake is using wheel radius instead of diameter, or mixing inches and centimeters. Always convert everything to the same unit before calculating. Also, the ratio alone does not tell you speed – you need wheel size and cadence. A 52/14 on a 700c road bike gives a different speed than the same ratio on a 20-inch BMX.

Another common misconception: a higher gear ratio always means faster. In reality, it means a higher theoretical speed at the same input RPM, but if you cannot turn the pedals (or the engine) at that RPM, you will be slower. The ratio only sets the relationship; your power and cadence determine actual speed.

Limitations of the Calculator

This calculator gives a theoretical speed based on perfect mechanical efficiency. Real-world speed is lower due to:

• Drivetrain friction (chain, bearings) – typically 2–5% loss
• Tire deformation and rolling resistance
• Wind resistance and rider weight
• Terrain gradient

It also assumes a constant cadence, which is rarely true. On a hill, your cadence drops unless you shift. For engine-driven vehicles, the calculation is similar but you replace cadence with engine RPM and include the final drive ratio. For example, if a car has a 3.5:1 first gear and a 4.1:1 differential, the total ratio is 3.5 × 4.1 = 14.35:1 – the wheels turn once for every 14.35 engine revolutions.

Alternatives and Related Calculations

If you are working with pulleys instead of gears, the same formula applies using pulley diameters instead of teeth. For belt drives, you use the pitch diameter. For gearboxes with multiple stages, multiply the individual ratios. If you want to know torque, multiply the input torque by the gear ratio (for reduction) or divide (for overdrive). For bike-specific metrics, you might also want gear inches or development (meters per pedal stroke). The calculator above gives you speed directly, but you can also use it to find the ratio needed for a target speed – just rearrange the formula.

Frequently Asked Questions

What is a good gear ratio for a bike?

For a road bike, a ratio of 2.5–3.0 (e.g., 50/19 to 50/16) is common for flat riding. For mountain bikes, lower ratios like 1.5–2.0 are better for climbing. The best ratio depends on your fitness, terrain, and cadence preference.

How do I calculate gear ratio from RPM and speed?

If you know the input RPM and the output speed, you can work backward. For a vehicle, divide the wheel RPM by the engine RPM to get the total ratio. For a bike, divide the wheel revolutions per minute by the cadence. You can use the formula: ratio = (speed × 5280 / 60) / (wheel circumference × cadence).

Does a higher gear ratio mean more speed?

At the same input RPM, yes – a higher ratio (drive/driven) gives more wheel rotations per input turn, so more speed. But it also requires more torque to accelerate. In practice, you may not be able to maintain the same RPM with a very high ratio.

What is the difference between gear ratio and gear inches?

Gear ratio is just the tooth count ratio (drive/driven). Gear inches multiply that ratio by the wheel diameter in inches, giving a number that represents the effective wheel size. For example, a 44/16 with a 26-inch wheel is 71.5 gear inches. Gear inches are useful for comparing bikes with different wheel sizes.

Can I use this calculator for a car or motorcycle?

Yes, with a modification. Replace cadence with engine RPM, and multiply by the final drive ratio (differential). The wheel diameter is the tire diameter. The speed formula works the same, but you must include the total gear reduction from engine to wheels.

Why does my actual bike speed differ from the calculator?

Because the calculator assumes 100% efficiency, constant cadence, and flat ground. Real-world losses from wind, rolling resistance, and drivetrain friction typically reduce speed by 5–15%. Additionally, your cadence varies with terrain and fatigue.

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in drive | Drive gear teeth | teeth | 44 | 1
in driven | Driven gear teeth | teeth | 16 | 1
in wheel | Wheel diameter | in | 26 | 1
in cadence | Cadence | rpm | 90 | 1
out ratio | Gear ratio | - | driven / drive | 2
out speed | Bike speed | mph | (drive / driven) * cadence * (wheel * pi / 12) * 60 / 5280 | 1