Torqmetry / Motorcycle Guide
Speedometer Accuracy & Tire Size Guide
Your speedometer and actual road speed can differ. The cause may be factory calibration, tire size, tire wear or where the motorcycle measures rotation.
Try the Speedometer Error Calculator →The essentials
- GPS comparison works best at a steady speed on an open, straight road; signal quality can affect the reading.
- A tire with a larger rolling circumference travels farther in one revolution, but the effect on a speedometer depends on how the bike senses speed.
- Nominal tire dimensions are estimates. Tire model, inflation, load and wear affect real rolling circumference.
Step-by-step
- 1
Compare indicated speed with GPS
Record the dashboard and GPS speeds at the same moment while traveling steadily and legally. Note the difference and its direction.
- 2
Record the old and new tire sizes
Write down width, aspect ratio and wheel diameter for each tire, such as 120/70-17.
- 3
Estimate diameter changes
Approximate overall diameter as rim inches × 25.4 + 2 × tire width in mm × aspect-ratio fraction.
- 4
Consider the speed sensor
Some motorcycles derive speed from transmission or wheel sensors. A tire change may not affect the speedometer in the same way on every model.
Understanding the calculation
Mode 1 — Speedometer vs GPS: Enter the speed your speedometer shows and the speed a GPS tracker records at the same moment. The calculator finds the difference as a percentage so you know how far off your speedo is.
Mode 2 — Tire size change: Enter your old and new tire dimensions (width, aspect ratio, rim diameter). The calculator computes the overall diameter for each: diameter = rim × 25.4 + 2 × (width × aspect / 100). The percentage difference in diameter equals the percentage change in your speedometer reading, because the speedo counts wheel revolutions. If the new tire is 3% larger, your actual speed is 3% higher than what the speedo shows.
Example: Up-sizing from 120/70-17 to 130/70-17
- 1. Old tire: width 120, aspect 70%, rim 17 in
- 2. Old diameter = 17 × 25.4 + 2 × (120 × 0.70) = 431.8 + 168 = 599.8 mm
- 3. New tire: width 130, aspect 70%, rim 17 in
- 4. New diameter = 17 × 25.4 + 2 × (130 × 0.70) = 431.8 + 182 = 613.8 mm
- 5. Difference = (613.8 / 599.8 − 1) × 100 = 2.3%
- 6. If your speedo reads 60 mph, your actual speed is 60 × 1.023 = 61.4 mph
Your actual speed is 2.3% higher than indicated (61.4 mph at a 60 mph reading)
Frequently Asked Questions
Why do motorcycle speedometers usually read high?
Most manufacturers calibrate speedometers to read slightly high to stay within legal tolerances and to account for tire wear. As a tire wears down, its diameter shrinks and the speedo reads even higher. This is normal and typically 5-10% fast.
Is GPS speed always more accurate than my speedometer?
GPS speed is generally very accurate on a straight, flat road at constant speed. It can be less accurate on tight corners, hills, or under heavy tree cover where satellite signals bounce. For the best comparison, measure on a long straight stretch at steady throttle.
Will changing tire width change my speedometer?
It is the overall diameter that matters, not width alone. A wider tire with the same aspect ratio has a larger diameter. But if you choose a lower aspect ratio to keep the same diameter (a "plus-sizing" swap), the speedometer may not change at all.
Do I need to recalibrate my speedometer after a tire change?
If the diameter difference is under 3% you usually do not need to recalibrate. For larger differences you can use a speedo healer or recalibration device to correct the reading. Some modern bikes can be recalibrated at the dealer.