How do I know if my car speedometer is accurate?

To know if your speedometer is accurate, do a practical test by comparing its reading with GPS-based speed readings at several steady speeds on a straight road. If you see a consistent difference, your speedometer may be off. Common causes include changes to tire or wheel size that alter the overall diameter, or instrument calibration issues that a dealership or qualified shop can correct.

How speedometers work and what can throw them off

The basics

Most modern speedometers calculate speed from a wheel-speed sensor and the vehicle’s gearing. The rotation rate of the wheels is translated into a road speed by the instrument cluster. Any change that alters the wheel’s effective circumference—such as aftermarket tires or oversized wheels—can throw the reading off relative to actual speed.

Common causes of inaccuracy

Beyond tire size changes, other factors include a faulty wheel-speed sensor, a worn or damaged instrument cluster, or a miscalibration that happened when tires were replaced or electronics were serviced. Weather, road conditions, and GPS signal quality can also affect how you perceive accuracy in a test, even if the speedometer itself is functioning within its normal range.

At-home checks you can perform

Use these simple tests to gauge whether your speedometer tracks actual speed. Each method has its own limits, so running multiple checks helps build a clearer picture.

• GPS comparison across speeds: On a long, straight road with clear sky view, set your cruise control to common speeds (for example, 30 mph, 50 mph, and 70 mph). Use a phone’s GPS-based speed readout or a dedicated GPS app, and compare it to what your dash shows. GPS readings can wander in tunnels or urban canyons, so choose open stretches if possible and take multiple measurements at each speed.


• Measured distance test (mile or kilometer): Mark a known distance (such as a measured mile). Drive at a steady target speed and time how long it takes to cover the distance, or simply read the speedometer while crossing the marker and compare to the expected speed. Do this a few times at different speeds to see if the discrepancy is consistent.


• OBD-II or app-assisted checks: If you have an OBD-II adapter and a speed-reading app, compare the “Vehicle speed” value shown there with GPS readings and your speedometer. Most vehicles’ OBD speed corresponds to actual wheel speed, so consistent differences point to calibration or tire-size issues rather than a faulty GPS.

Conclusion for this section: If you observe a consistent discrepancy at multiple speeds that exceeds a few percent, your speedometer is likely off. Factors such as tire size changes are the most common cause, but professional calibration can correct it.

What to do if you find your speedometer is off

When you have evidence of inaccuracy, consider these steps to determine the right course of action.

• Verify tire and wheel sizes: Check the label on the driver’s door jamb or owner’s manual for the stock tire size, and compare it with what’s currently installed. If you’ve recently changed tires or installed aftermarket wheels, the diameter may be different enough to affect readings.


• Calculate the potential error from tire size changes: A larger overall diameter tends to make the speedometer read low (you’re going faster than the gauge shows); a smaller diameter tends to make it read high. A rough rule of thumb is that a circumference change of about 1–2% will produce roughly a 1–2% speedometer error, with larger changes producing bigger errors.


• Consider recalibration or programming: Many modern vehicles can be recalibrated by a dealer or qualified shop to account for non-stock tire/wheel sizes. Some models allow you to input the tire size in the vehicle’s settings or through a service tool, which adjusts the speedometer accordingly. In other cases, the instrument cluster or ECU may require professional reprogramming or replacement.


• Investigate potential sensor or wiring issues: If the speedometer is erratic or has dead zones, a faulty wheel-speed sensor, damaged wiring, or a failing cluster could be the culprit and require diagnostic testing.

Note: Not all vehicles offer easy at-home calibration, and some require specialized tools or software. If in doubt, consult your dealership or an automotive technician to avoid inadvertent safety or legal issues related to incorrect speed readings.

Safety, legality, and practical tips

Speedometer accuracy matters for safe driving and for legal compliance when traveling at posted limits. When testing, use plenty of space, stay in safe conditions, and remember GPS readings can be imprecise in certain environments. If you routinely drive with aftermarket tires, expect some variance and verify the manufacturer’s guidance on speedometer compensation for your setup.

Summary

In short, you can check your speedometer’s accuracy by comparing its readings with GPS measurements at multiple speeds and by conducting a measured-distance test. If there’s a consistent deviation, investigate tire size changes as the most common cause, then pursue recalibration with a dealer or qualified shop if needed. Regular checks are a good habit to ensure your speed readings remain reliable for safe driving and legal compliance.

How can I tell if my speedometer is correct?

Compare the GPS reading to your speedometer — note any difference. If you want to double-check, you can do a timed mile: use roadside distance markers, note the time it takes to cover a mile at a steady speed, then calculate your true speed (speed = distance ÷ time).

Do car speedometers show actual speed?

No, car speedometers are generally not perfectly accurate, as they are intentionally designed to read slightly higher than the actual speed. This is a safety measure to prevent drivers from accidentally exceeding the speed limit and to protect manufacturers from liability in case a driver is speeding. Factors like changes in tire size, tire wear, and pressure can also impact their accuracy. 
Why speedometers are inaccurate

• Legal requirements: Regulations in many countries require that a speedometer never show a speed lower than the actual speed. To comply, manufacturers set the speedometer to read higher to ensure it remains a safety buffer. 
• Safety: Manufacturers calibrate speedometers to err on the side of caution, overstating the speed by a few percent to reduce the risk of a driver speeding unintentionally. 
• Manufacturing and calibration: It's difficult to produce a perfectly accurate speedometer for every single vehicle due to variations in manufacturing. Speedometers are calibrated to work with the original factory tire size. 
• Tire and wheel changes: When you change tires or wheels, you change the overall diameter. If the new tires have a different circumference, your speedometer's reading will be affected, as it measures speed based on the rate of tire rotation. 

How to check your speedometer's accuracy

• Use a GPS: A smartphone app like Google Maps or Waze, or a dedicated GPS device, can show you your actual speed. Drive at a steady speed and compare the GPS reading to your speedometer's reading. 
• Do a timed mile: Find a clear, straight road. Use your car's speedometer to set a steady speed and time how long it takes to cover a measured mile. You can then calculate your actual speed. 
  • Formula: Actual Speed = (60 / time in minutes) * 1.609 (for miles) or (60 / time in minutes) * 1.609 (for km). 

How do I get my speedometer checked?

Typically, mechanics will perform a speedometer calibration test with a dynamometer. The dynamometer will determine your vehicle's actual speed by examining your tire rotation. The mechanic will document your vehicle's actual speed and your vehicle's displayed speedometer speed.

How accurate is the speedometer on a car?

Car speedometers are designed to read slightly higher than your actual speed for safety, though they are still fairly accurate, usually to within a few mph. Factors like tire wear, size, and pressure can affect accuracy, so using a GPS for real-time speed comparison is often a good way to gauge your car's specific speedometer.
 
How accurate are car speedometers?

• Deliberately slightly fast: Most manufacturers intentionally calibrate speedometers to overstate the actual speed. This is a safety measure to help prevent drivers from unintentionally speeding and to reduce liability. 
• Regulatory allowance: Regulations in both the US and EU allow for a certain margin of error, typically meaning a speedometer must never show a speed lower than the actual speed. 
• Actual accuracy: When new and with factory-standard tires, a car's speedometer is usually accurate within 1-2 mph. 

What affects speedometer accuracy?

• Tire size: The most common cause of inaccuracy is using tires that are not the same size as the factory-installed ones. Different tire sizes have different circumferences, which affects the speedometer reading. 
• Tire pressure and wear: Under-inflated or over-inflated tires can significantly impact the readout. Worn tires also have a smaller diameter, which can cause the speedometer to show a higher speed than actual. 
• Wheel changes: Changing the wheels to a non-factory size can also impact accuracy. 

How to check your speedometer's accuracy

• Use a GPS app: The most reliable method is to compare your car's speedometer to a GPS app on your phone or a dedicated GPS device. GPS systems measure your speed based on your actual movement over the ground and are generally more accurate for this purpose. 
• Timed mile: A traditional method is to use a stopwatch and time how long it takes to travel a mile marker on the highway. You can then calculate your actual speed using the formula: speed=distancetimes p e e d equals the fraction with numerator d i s t a n c e and denominator t i m e end-fraction𝑠𝑝𝑒𝑒𝑑=𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒𝑡𝑖𝑚𝑒.