Do car speedometers show actual speed?
Yes, speedometers are designed to indicate how fast a vehicle is moving, but they are not perfectly precise.
In most cars, readings stay within a small margin of error, yet can diverge from true speed due to factors like tire size, calibration, and sensor tolerances. This article explains how speedometers work, why readings can differ from reality, and what you can do to verify or adjust accuracy.
How speedometers work
Most modern cars determine speed from a vehicle speed sensor (VSS) located in the transmission or from wheel-speed sensors tied to the anti-lock braking system. The sensor counts wheel revolutions and, using the vehicle’s gear ratio and the tires’ circumference, the onboard computer estimates ground speed. The dashboard then displays that estimate. Older models used a mechanical cable directly connected to a speedometer gauge. In contemporary vehicles, the signal is processed by the car’s computer and shared with features such as cruise control and stability programs.
Accuracy and what affects it
Understanding the factors that influence speedometer accuracy helps explain why readings can diverge from true speed.
- Tire size and type: The speedometer assumes a specific tire diameter. Replacing with larger or smaller tires changes the actual distance traveled per wheel rotation, shifting the indicated speed relative to true speed.
- Tire wear and inflation: Worn tires or incorrect air pressure alter effective diameter and rolling behavior, nudging readings away from reality.
- Wheel/tire combos and aftermarket gear: Aftermarket wheels or tires with nonstandard circumferences can introduce additional error.
- Calibration and sensor tolerance: The speed sensor and instrument cluster have manufacturing tolerances and can drift over time, especially in extreme temperatures.
- Vehicle load and suspension: Heavier loads can subtly affect tire shape and contact patch, modestly influencing diameter measurements.
- Regulatory tolerances and design goals: Automakers design speedometers to be close to true speed, but they are not calibrated to be perfect at every speed and condition.
In practice, the typical deviation is a few percent, with larger discrepancies more likely when tire size changes or the tires wear unevenly. A speedometer that reads slightly higher or lower than actual speed is common across many makes and models.
What to do if you suspect inaccuracy
If you’re concerned about speedometer accuracy, a simple, cautious check and potential recalibration can help ensure you’re not unknowingly speeding or under-speeding.
- Check that your tire size matches the vehicle’s specification. Tire size is listed on the door placard and in the owner’s manual; aftermarket tires with a different diameter will affect readings.
- Test against GPS-based speed for a short drive over a measured distance. Use a reliable GPS app or device and compare it to the dashboard reading at a steady speed.
- Inspect tire inflation and wear. Ensure tires are inflated to the recommended pressure and inspect for uneven wear that could alter effective diameter.
- Review whether your vehicle allows speedometer recalibration. Some cars let dealers adjust the displayed speed in the instrument cluster or via software; others require more involved service.
- When changing tire sizes or wheels, plan for recalibration if the OEM circumference changes significantly.
Not all vehicles permit easy user calibration; if you suspect a persistent error, consult the manufacturer or a qualified technician. Recalibration can improve accuracy, but it may not be available on every model.
Summary
Speedometers do not measure speed with perfect precision; they estimate speed from wheel rotation and drivetrain data. Their readings are generally close to actual speed but can diverge due to tire size, wear, inflation, calibration, and other factors. Owners can verify accuracy by cross-checking with GPS and, if needed, seek professional recalibration or tire-size adjustments to restore alignment between indicated and true speeds.
