Bicycle Gear Speed Calculator

Assumptions & limitations (why real speed may differ)

• Flat ground, no wind: This calculator converts gearing + cadence into theoretical ground speed. It does not model headwind/tailwind, drafting, or aerodynamics.
• No climbing/descending: Road gradient changes the power required to hold a cadence, but the gearing-to-speed math itself is unchanged.
• 100% drivetrain efficiency: Chain friction, pulley losses, and bearing drag are ignored, so real-world speed may be slightly lower at the same cadence.
• Wheel size input must be an effective diameter: Tire labels (e.g., “700c”, “29er”) are not literal diameters in meters. For best accuracy, use the measured rolling circumference (rollout test) and convert to diameter (diameter = circumference ÷ π).
• Tire deformation & pressure: The effective rolling radius changes with tire pressure, rider weight, and surface, so the same tire can produce different results.
• Indoor trainers: Smart trainers may report speed differently depending on calibration and virtual tire circumference settings.

Formula (variables defined)

Per crank revolution, the rear wheel turns by the gear ratio, so the distance per pedal stroke (rollout) is wheel circumference times gear ratio. Using wheel diameter in meters:

Where d is distance per crank revolution (m/rev), w is wheel diameter (m), C_r is chainring teeth, and C_c is rear cog teeth.

With cadence cad in RPM, speed in meters per minute is d × cad. Convert to km/h by multiplying by 60 and dividing by 1000; convert to mph by multiplying km/h by 0.621371.

FAQ

Why is my GPS speed different from the calculator?

GPS is affected by signal quality and smoothing, and real riding includes wind, hills, tire slip, and drivetrain losses. This tool outputs the theoretical speed from gearing and cadence only.

Is “700c” equal to 0.70 m wheel diameter?

No. “700c” is a sizing standard, not a literal diameter. Effective rolling diameter depends on rim, tire width, and tire pressure. Measuring rollout is the most accurate approach.

What cadence should I use?

Many recreational riders cruise around 60–90 RPM; trained riders often prefer ~80–100 RPM on flats. Use the cadence you can comfortably sustain for the riding you’re modeling.

What’s the difference between gear ratio, rollout, and gear inches?

Gear ratio is chainring/cog. Rollout is distance per crank revolution. Gear inches is gear ratio × wheel diameter (in inches), useful for comparing “how hard” a gear feels across bikes.