* Tire Size Discrepancy: This is the most common cause. If you've changed to larger-diameter tires (taller sidewalls, larger rims, or both) than the factory-specified size, your speedometer will read faster. Larger tires rotate fewer times per mile than smaller tires, leading to an underestimation of the distance traveled and an overestimation of speed. Conversely, smaller tires will make the speedometer read slower.
* Transmission Issues: Problems within the transmission, such as a faulty speedometer gear or sensor, can cause inaccurate readings. This is less likely to consistently read *faster*, but it's a possibility.
* Speed Sensor Malfunction: The vehicle speed sensor (VSS) is responsible for sending speed data to the instrument cluster. A malfunctioning VSS can cause inaccurate readings, either faster or slower. This is another possibility that can't be ruled out.
* Incorrect Gear Ratio: While less common, an incorrect final drive gear ratio in the differential can also affect speedometer accuracy. This would usually be a result of a repair or modification that wasn't done correctly.
* Calibration Issues (less likely): It's possible, though less probable, that the speedometer itself needs calibration. This is usually not something that happens on its own and is more common after repairs or modifications to the instrument cluster.
How to Diagnose:
1. Check Tire Size: Compare your current tire size to the factory specifications listed in your owner's manual or on a sticker usually located on the driver's side doorjamb. If they differ, this is almost certainly the cause.
2. GPS Verification: Use a GPS device (smartphone app, dedicated GPS unit) to compare the speed shown on your speedometer to the actual speed. A consistent and significant difference points to a problem.
3. Professional Diagnosis: If the tire size is correct, a mechanic can use diagnostic tools to check the VSS, transmission, and other relevant components to pinpoint the cause of the inaccurate reading. This is recommended if the GPS confirms the speedometer is consistently and significantly inaccurate.
In most cases, the tire size is the culprit. Once you determine the correct tire size, you can either adjust the speedometer calibration (using a tool that adjusts the signal to the speedometer) or accept a small amount of error. Many people choose to simply use a GPS for accurate speed information.
