How Does a Bike Speedometer Work? | Wheel Rotation To Speed

A magnet on one spoke passes a sensor on the fork once per wheel turn, and the computer converts those pulses into current speed and total distance. The system is simple, reliable, and has helped cyclists pace themselves for decades. Here is how the calculation works, what each part does, and how to set yours up without common mistakes.

What Each Part Does

Every standard bike speedometer has three components, each handling one job:

• Computer head unit — mounted on the handlebar, performs calculation and displays speed, distance, and trip time.
• Sensor — attached to the fork or frame. It detects the magnet passing and sends a pulse.
• Spoke magnet — bolted or taped to one spoke so it passes the sensor once per wheel rotation.

Older models use a reed switch; newer sensors use Hall-effect detectors that last longer. One pass = one pulse. Modern sensors can send data wirelessly via ANT+ or Bluetooth to a bike computer, smartwatch, or phone app, but the base measurement stays identical: the sensor still counts wheel turns.

The Formula That Converts Rotations To Speed

Stored wheel circumference (distance in one full turn) multiplied by revolutions per unit time gives speed: Speed = (Revolutions × Wheel Circumference) ÷ Time. For a 700c road bike, rolling circumference is about 2,100 mm (83 inches). If the sensor counts 200 revolutions in 60 seconds, the computer does 200 × 83 inches = 16,600 inches per minute, then converts to mph. The calculation updates several times per second. The same pulse counting produces total distance: 1,000 wheel turns on that tire gives 1.3 miles traveled. If wheel circumference is wrong, every number displayed is off by the same percentage.

How To Install And Calibrate A Bike Speedometer

Setup takes about 15 minutes with a small Phillips screwdriver or hex key:

1. Mount the sensor on the fork. Position it directly in line with where the magnet will pass, leaving no more than 2–5 mm clearance.
2. Attach the magnet to a spoke. Fasten it on the same side as the sensor, about level with the sensor center. Spin the wheel and confirm the magnet passes with that small gap.
3. Secure all cabling. Zip-tie wires along the fork and brake cable housing so nothing flops into the wheel or spokes.
4. Input the wheel circumference. Use the manual roll-and-measure method (mark tire on pavement, roll one revolution, measure between marks) — it beats generic tire charts because actual tire width and pressure change true rolling circumference.
5. Set the units — miles or kilometers.

The most common failure point is magnet-sensor alignment. Even a 1 mm gap beyond recommended clearance can cause intermittent readings. If you want a durable wired model, see our tested picks for wired bicycle speedometers.

Common Setup Mistakes That Ruin Accuracy

• Wrong wheel circumference. The biggest accuracy killer. Entering 2,150 mm instead of 2,095 mm shifts speed by 2.6%.
• Poor magnet-sensor alignment. Gap exceeding 5 mm or skewed alignment causes missed pulses and wild speed jumps.
• Parts interfering with wheel movement. Sensor too close to brake rotor or cable caught in spokes can damage components. Spin the wheel freely before your first ride.

Compatibility is straightforward: any standard bicycle with spokes and a fork can mount a magnet-and-sensor speedometer. The only edge case is carbon forks, which sometimes require a different sensor mount.