Changing your vehicle’s tires can sometimes lead to unexpected issues, one of the most common being an inaccurate speedometer. If you’ve recently upgraded your tires, especially to a different size than stock, you might have noticed that your speedometer doesn’t quite match your actual speed. This discrepancy occurs because tire size directly impacts the revolutions per mile your wheels make, which is what your speedometer uses to calculate speed. Driving with an incorrect speedometer isn’t just inconvenient; it can lead to speeding tickets and inaccurate mileage readings, affecting your car’s service intervals and resale value.
Understanding Speedometer Calibration and Tire Size
The factory speedometer setting in your car is calibrated for the original tire size. When you switch to tires with a different diameter, the rolling circumference changes. Larger diameter tires cover more ground per revolution, meaning your car is actually traveling faster than your speedometer indicates. Conversely, smaller diameter tires mean you’re going slower than what’s displayed. This is why after a tire change, especially when upsizing or downsizing, speedometer recalibration becomes essential.
Traditionally, correcting a speedometer required a visit to a dealership or a specialized mechanic. However, with the advent of OBD2 (On-Board Diagnostics II) scanners and user-friendly apps, you can now take control of this process yourself. So, Can You Use Obd2 To Change Speedometer settings? The answer is yes, and it’s more accessible than you might think.
OBD2 Scanners and Speedometer Adjustment: A DIY Approach
OBD2 scanners are diagnostic tools that plug into your car’s OBD2 port, typically located under the dashboard. These scanners, when paired with the right software, can access and modify various vehicle settings, including tire size parameters that affect speedometer readings.
For Jeep Patriot owners and potentially other makes and models, apps like JScan have emerged as powerful tools for DIY speedometer calibration. These apps communicate with your car’s computer via an OBD2 scanner, allowing you to input your new tire size and adjust the speedometer accordingly. This eliminates the need for expensive dealership visits and puts the control back in your hands.
Tools You’ll Need for OBD2 Speedometer Recalibration
To recalibrate your speedometer using an OBD2 scanner, you’ll need the following:
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OBD2 Scanner: A Bluetooth OBD2 scanner that is compatible with your smartphone and the JScan app (or similar app for your vehicle make). Popular and affordable options include:
- Veepeak Bluetooth OBD2 Scanner: https://www.amazon.com/Veepeak-Bluetooth-Diagnostic-Supports-Vehicles/dp/B076XVQMVS/
- Vgate Bluetooth OBD2 Scanner: https://www.amazon.com/Vgate-Bluetooth-Scanner-Android-Compatible/dp/B06XGB4873
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Smartphone or Tablet: An Android or iOS device to run the JScan app (or equivalent).
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JScan App (or Vehicle-Specific App): Download and install the JScan app. Note that a license purchase (around $20) is typically required to access modification features.
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Tape Measure: For accurately measuring your actual tire diameter.
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Tire Size Calculator (Optional): Online tools like Tire Size Converter can give you an estimated tire diameter, but actual measurement is recommended.
Step-by-Step Guide: Changing Speedometer Settings with OBD2 and JScan
Here’s a detailed walkthrough on how to use an OBD2 scanner and the JScan app to adjust your speedometer for accurate readings after a tire change:
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Connect the OBD2 Scanner: Plug your OBD2 scanner into your vehicle’s OBD2 port. This is commonly found under the dashboard on the driver’s side.
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Pair with JScan App: Turn on your vehicle’s ignition (you don’t need to start the engine). Open the JScan app on your smartphone or tablet and follow the app’s instructions to connect to your OBD2 scanner via Bluetooth. You may need to purchase and activate a JScan license if you haven’t already.
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Measure Your Tire Diameter: While online calculators can provide estimates, the most accurate method is to measure your tires directly. Use a tape measure to get the diameter from the top to the bottom of your tire when it’s mounted on your vehicle and properly inflated. Record this measurement in inches.
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Navigate to Tire Size Settings in JScan: Once connected, navigate within the JScan app to find the tire size or speedometer calibration settings. The exact location may vary slightly depending on your vehicle and the app version, but look for options related to “PCM Configuration,” “Body Control Module,” or similar systems that control vehicle parameters.
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Enter Your Measured Tire Diameter: JScan may display your current tire size setting (likely the stock diameter). Input the tire diameter you measured in the previous step. The app may allow you to enter the diameter in inches or millimeters.
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Apply and Save Changes: After entering the new tire size, follow the app’s prompts to apply and save the changes to your vehicle’s computer. This process may take a minute or two, so be patient and do not exit the app or disconnect the OBD2 scanner during this time.
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Verify Speedometer Accuracy: Once the changes are applied, start your vehicle and take it for a test drive. Use a GPS app on your smartphone or a GPS device to compare your actual speed with your speedometer reading. Ideally, they should now be very close. You can also use a speedometer calibration app for more precise verification.
Important Considerations and Cautions
- Connection Issues: Bluetooth OBD2 scanners can sometimes be finicky. If you encounter connection problems, try restarting the app, disconnecting and reconnecting the scanner, or ensuring your vehicle’s ignition is on.
- Wait for Processing: When applying changes in JScan, allow sufficient time for the process to complete. Do not interrupt the process, as this could potentially cause issues.
- Potential for Glitches: While generally safe, modifying vehicle settings with OBD2 scanners carries some inherent risk. Be cautious and only change settings you understand. If you experience any unexpected issues like warning lights after making changes, double-check your settings and consider reverting to the original configuration. In the original article, the author mentioned a temporary ABS light issue that resolved itself, but it’s a reminder to proceed with care.
- App Compatibility and Vehicle Support: Ensure that the JScan app (or alternative app) supports your specific vehicle make, model, and year before purchasing a license and making changes. Not all vehicles are supported, and features may vary.
- Professional Assistance: If you are uncomfortable performing these steps yourself or encounter difficulties, it’s always best to consult a qualified mechanic or your dealership for speedometer calibration.
Conclusion: DIY Speedometer Correction is Achievable
Correcting your speedometer after a tire change using an OBD2 scanner and an app like JScan is a viable and cost-effective DIY solution. By following these steps and exercising caution, you can ensure your speedometer and odometer are accurate, contributing to safer driving and proper vehicle maintenance. Taking control of your car’s settings with OBD2 tools empowers you to handle common modifications and maintenance tasks, saving you time and money while enhancing your understanding of your vehicle.
