What Causes a Speedometer To Be Inaccurate?
You check your speedometer, then glance at your phone’s GPS, and the two numbers don’t match. That mismatch might leave you second-guessing whether you’re actually within the speed limit.
At 5 Star Tuning, we carry performance tuners for trucks, Jeeps, SUVs, and more. The crew can help drivers make sense of the speedometer issues that may show up after upgrades or part failures. In this guide, we will explain what causes a speedometer to be inaccurate and how to fix the mismatch.
How a Speedometer Gets a Reading
If you would like to understand how speedometers work, start with the way your vehicle measures movement. Modern vehicles may use a vehicle speed sensor, such as a transmission output shaft sensor or an ABS wheel speed sensor, to track how fast key rotating parts are moving while you drive. Those sensors send speed-related data to the vehicle’s computer, which flashes the corresponding number on the dashboard.
In older vehicles, you might see a mechanical cable connected to the transmission that performs a similar job.
Possible Reasons for Your Vehicle’s Mismatched or Inaccurate Speedometer Reading
What causes a speedometer to be inaccurate? A few considerations will guide your tuning team’s adjustments.
Speedometer Calibration
Your speedometer’s calibration at the factory is based on an exact tire diameter. Once the rolling diameter changes, the speed reading may shift, even if the speedometer isn’t broken.
Tire Size
What causes a speedometer to be inaccurate sometimes comes down to changing your tires. Larger tires cover more ground per rotation. If your original tires were smaller, your speedometer may show a slower reading than your actual speed.
Sensor and Wiring Problems
A failing sensor or damaged wiring can also throw off your speedometer’s readout. Faulty wiring between the sensor and the dashboard can interrupt the signal, causing the speedometer to flicker or display numbers that don’t match your actual speed. A few signs tend to show up alongside these electrical issues, including:
- Erratic needle movement
- Check engine light activation
- Intermittent readings
Your tuner will need to pinpoint which part or wire is causing the issue and fix it.
Transmission
What causes a speedometer to be inaccurate? In many vehicles, the speed sensor sits at or near the transmission output shaft. If the sensor or drive gear is faulty, the speed signal that reaches the dashboard may not be accurate.
Instrument Cluster Issues
If the instrument cluster’s internal electronics fail, the display may show the wrong speed or stop responding. This can happen even when the tires are the original size, and the speed sensors are working properly.
Get Better Speedometer Accuracy With 5 Star Tuning
Now you know what causes a speedometer to be inaccurate, and you can better understand why a change in tire sizing can be a big deal during an upgrade. If you want to learn more about related performance issues, including the causes of engine knock, or explore tuners and calibration tools after a tire or wheel upgrade, we can help. Give 5 Star Tuning a call at (843) 536-1244 to discuss speedometer calibration support in Florence, SC.
