Changing your vehicle’s tire size can impact the accuracy of your speedometer. This article will guide you through how to use an OBD2 scanner and the JScan app to recalibrate your speedometer, ensuring accurate readings after a tire change. If you’ve recently swapped out your tires and noticed your speedometer seems off, you’re in the right place to learn how to fix it yourself using readily available tools.
It’s a common scenario: you decide to upgrade your tires for better performance or aesthetics. In the original article, the author replaced their stock tires on a Jeep Patriot, moving from 235/55/r17 to 225/65/r17 all-terrain tires. This change, while beneficial for off-road capability, led to a speedometer discrepancy. The vehicle’s computer, calibrated for the original tire size, was now miscalculating the speed based on the new tire dimensions. In this case, the speedometer was underreporting the actual speed, showing 60 mph when the vehicle was traveling at 64 mph.
This discrepancy isn’t just a minor inconvenience. An inaccurate speedometer can lead to:
- Speeding tickets: Thinking you’re within the speed limit when you’re actually exceeding it.
- Inaccurate mileage tracking: Affecting fuel economy calculations and service intervals.
- Potential safety issues: Inconsistent speed readings can be confusing and potentially dangerous in certain driving situations.
Fortunately, modern vehicles equipped with an On-Board Diagnostics II (OBD2) system offer a solution. With the right tools, you can recalibrate your speedometer to match your new tire size. This guide will walk you through the process using an OBD2 scanner and a popular app called JScan, mirroring the experience of the original article’s author.
Tools You’ll Need for OBD2 Speedometer Calibration
To perform an Obd2 Speedometer Calibration at home, you will need the following:
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OBD2 Scanner: This device plugs into your vehicle’s OBD2 port and communicates with your car’s computer system. There are many affordable options available online. The original article mentions two examples available on Amazon:
These scanners are Bluetooth-enabled, allowing them to connect wirelessly to your smartphone.
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JScan App: This is a mobile application (available for Android and iOS) specifically designed for Jeep, Chrysler, Dodge, and Ram vehicles. It provides advanced diagnostic and customization options, including speedometer calibration. While JScan requires a license purchase (approximately $20 as mentioned in the original article), it offers a user-friendly interface for making these adjustments.
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Smartphone or Tablet: To run the JScan app and connect to the OBD2 scanner via Bluetooth.
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Tape Measure: To accurately measure the actual height of your new tires. While online tire size calculators can provide estimates, real-world measurements are more precise for calibration.
Step-by-Step Guide to OBD2 Speedometer Calibration with JScan
Once you have gathered the necessary tools, follow these steps to recalibrate your speedometer:
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Install the JScan App and Purchase a License: Download the JScan app from your device’s app store and follow the in-app instructions to purchase a license. This license is essential to access the calibration features.
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Connect the OBD2 Scanner: Locate the OBD2 port in your vehicle. In the original article, the author mentions it being “between my knee and my foot above the accelerator pedal” in their UK Jeep Patriot. Plug your OBD2 scanner into this port.
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Pair the OBD2 Scanner with JScan: Turn your vehicle’s ignition to the “ON” position (you don’t need to start the engine). Enable Bluetooth on your smartphone or tablet and open the JScan app. Follow the app’s instructions to connect to your OBD2 scanner. This may involve selecting your scanner from a list of available Bluetooth devices. The original author noted some connection issues, suggesting trying to reconnect, restarting the app, or ensuring the engine is on if needed.
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Measure Your Tire Diameter: Use a tape measure to determine the actual diameter of your new tires. Measure from the very top to the very bottom of the tire while it is mounted on your vehicle and inflated to the recommended pressure. It’s important to get an accurate measurement as this value will be used for calibration. The original article highlights the difference between calculated tire size and actual measured size, emphasizing the importance of physical measurement.
Alt Text: Measuring the diameter of a vehicle tire with a tape measure from top to bottom to ensure accurate input for speedometer calibration using the JScan app.
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Enter Tire Size in JScan: In the JScan app, navigate to the speedometer calibration or tire size adjustment section. The exact menu names may vary slightly depending on the JScan app version and your vehicle. You will likely be prompted to enter the tire diameter in inches. Input the measured tire diameter you obtained in the previous step. The original author found that their measured tire size was slightly different from online calculators and adjusted accordingly within JScan.
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Apply and Wait: Once you have entered the new tire diameter, confirm the changes in the JScan app. The app will then communicate with your vehicle’s computer to update the speedometer settings. The original author noted that this process took 1-2 minutes, so be patient and do not close the app or disconnect the OBD2 scanner during this time.
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Verify Calibration: After the process is complete, it’s crucial to verify the accuracy of your speedometer. Use a GPS-based speedometer app on your smartphone or a standalone GPS device to compare its speed reading with your vehicle’s speedometer at various speeds. The original author used GPS on Google Maps and the “Car Scanner ELM OBD2” app to confirm the accuracy.
Important Considerations and Potential Issues
- OBD2 Scanner Compatibility: Ensure that the OBD2 scanner you purchase is compatible with your vehicle’s make and model. While OBD2 is a standard, some scanners may have better compatibility than others.
- JScan App Compatibility: JScan is specifically designed for Jeep, Chrysler, Dodge, and Ram vehicles. If you have a different make, you will need to research alternative apps that offer speedometer calibration for your vehicle.
- Connection Issues: As mentioned in the original article, you might encounter Bluetooth connection issues between your smartphone and the OBD2 scanner. Troubleshooting steps include restarting the app, reconnecting the scanner, and ensuring your vehicle’s ignition is in the “ON” position.
- ABS Light: The original author briefly experienced an ABS light after making changes with JScan. This could be a temporary glitch, but it’s important to be cautious when using such apps. Only change settings you understand and are confident in. If you encounter persistent warning lights, consult a professional mechanic.
- Software Glitches: While rare, software glitches can occur. If you experience unexpected issues after using JScan, it may be helpful to disconnect the OBD2 scanner, restart your vehicle, and check if the issue resolves itself.
Conclusion
Recalibrating your speedometer after changing tire sizes using an OBD2 scanner and the JScan app is a straightforward process that empowers you to maintain accurate speed readings and mileage tracking. By following these steps and taking the necessary precautions, you can ensure your speedometer is correctly calibrated for your new tires, enhancing driving safety and accuracy. This DIY approach not only saves you money compared to dealership recalibration but also provides a valuable understanding of your vehicle’s systems. Remember to always double-check your measurements and proceed with caution when using OBD2 apps to modify vehicle settings.
