Is the speedometer on a car accurate?
In most cars, the speedometer is accurate within a small margin—typically a few percent of actual speed. The exact error varies by vehicle, tire size, wear, and driving conditions.
Speed readings are produced by a sensor that tracks wheel or transmission speed and then a calibration in the instrument cluster translates that rotation rate into a headed speed. Changes to tires, wheels, or load can shift the reading, and understanding these factors helps explain why your speedometer might read differently from GPS or from other measurements.
How speedometers work and what affects accuracy
Speedometers rely on a sensor that counts wheel revolutions or transmission output shaft turns. The vehicle’s computer applies a calibration constant based on the assumed tire diameter and drivetrain gear ratios to display speed in mph or kph. If the actual tire circumference differs from what the system expects, the displayed speed will diverge from your true speed.
How it’s calibrated
The calibration is built around stock equipment: tire size, wheel diameter, and final-drive ratio. When everything matches the factory spec, the indicated speed is designed to be very close to actual speed within a manufacturer-specified tolerance. If you change any of those core inputs—especially tire size—the calibration can become less accurate unless adjusted.
What usually causes inaccuracy
Several common factors can cause the speedometer readout to drift from real speed. Tire size and wear are the biggest contributors, followed by incorrect tire pressure, aftermarket wheels, and any software or sensor tolerances baked into the system. Seasonal temperature changes and load (carrying heavy cargo or passengers) can also affect tire diameter slightly and alter readings.
Common factors that affect accuracy
Here is a concise rundown of the main factors that influence how closely the speedometer tracks actual speed.
- Changes in tire size or circumference (stock tires vs. aftermarket or larger/smaller tires)
- Tire wear and incorrect inflation pressure
- Aftermarket wheels or non-standard wheel/tire combinations
- Speedometer calibration and sensor accuracy, including manufacturing tolerances
- Vehicle load and ride height (heavier loads or sagging suspensions can alter effective diameter)
- Drivetrain gear ratios and final drive (typically fixed, but some vehicles allow reprogramming)
Understanding these factors helps explain why readings can differ between vehicles or under different driving conditions.
How to test and correct accuracy
To verify speedometer accuracy, you can compare the reading against a GPS-based speed readout on a safe, controlled stretch of road.
- Choose a clear, straight segment of road with visible mile markers and minimal traffic.
- Record indicated speed and GPS speed at several steady speeds (for example 30 mph, 50 mph, and 70 mph) over a few miles.
- Note whether the speedometer reads higher or lower than the GPS throughout the test and by how much (percentage or mph).
- Check your tires’ size and pressure against the manufacturer’s specifications. If you’ve installed aftermarket tires or wheels, calculate the new approximate circumference to estimate expected error.
- If a substantial discrepancy exists, consult a dealership or qualified shop. Some vehicles allow recalibration or software updates to adjust the speedometer for current tires; others may require hardware changes or reprogramming.
While a small discrepancy is common and often harmless, a significant bias should be addressed to ensure accurate speed readings, reliable odometer data, and proper compliance with speed laws.
Summary
Speedometers are designed to be accurate within a modest tolerance, but accuracy can vary due to tire size, wear, pressure, aftermarket wheels, and calibration. If you suspect an issue, compare your speedometer to GPS readings across a range of speeds, verify tire specifications, and consult a professional if a large discrepancy appears. In everyday driving, a small margin is normal, but for precise speed tracking or odometer accuracy, verification and, if needed, recalibration are worthwhile steps.
