Why Your Car's Speedometer May Not Match Actual Highway Speeds

A common frustration for many motorists, especially those driving at high speeds on highways for extended periods, is the discrepancy between their speedometer reading and the actual speed they are traveling. While the speedometer is a useful tool, it is not a precision scientific instrument and can deviate from the true speed.

Speedometer Accuracy vs. Precision

Your car’s speedometer is designed to give you a close approximation of the speed you are traveling, but it is not always perfectly accurate. Factors such as atmospheric pressure, temperature, and tire size can affect the readings shown on the speedometer. A frequency observed across many vehicles is the use of non-standard tire sizes, which can change the number of rotations it takes to travel a mile. For example, tires with a 55 sidewall ratio might provide slightly different speed readings compared to those with a 50 or 60 sidewall ratio. Choosing between these sizes can impact your speedometer readings, so it is the driver's responsibility to ensure the speedometer is calibrated correctly.

Why Speedometers Overread

Speedometers are typically calibrated to read higher than the actual speed for a very good reason. In most legal jurisdictions, it is illegal for a speedometer to indicate a lower speed than the actual speed. Calibration to overread prevents the speedometer from being out of calibration by a significant amount, thus giving a more accurate overall reading.

A speedometer set precisely to the actual speed would require frequent recalibration to maintain accuracy, which would be impractical from a legal and operational standpoint. Additionally, if the speedometer were accurate, even the slightest momentary increase in speed—like during acceleration—could result in a higher reading, making it more likely for drivers to exceed legal speed limits. By overreading, the speedometer helps mitigate the risk of accidental or momentary speeding, thus reducing the likelihood of receiving speeding tickets.

Calibration and Responsibility

As the driver who chooses the tire size, you are legally responsible for ensuring that the speedometer is calibrated correctly. If you have made changes to the wheel or tire size, you should consult the vehicle manufacturer or a professional to recalibrate the speedometer. Failure to do so can lead to potential legal issues and may impact your driving experience and safety.

For accuracy, it's also recommended to use a GPS device, which provides a more precise measurement of your speed. GPS devices are particularly useful at high speeds and for extended highway driving, as they can provide real-time updates and help ensure you stay within legal limits.

In conclusion, while your car's speedometer is a useful tool, it is important to understand that it is not a precision scientific instrument and may not always match the actual speed. Factors such as tire size, atmospheric conditions, and legal standards contribute to this discrepancy. Regular calibration and the use of additional technology, like GPS, can help improve accuracy and ensure safe and legal driving practices.