So, you’ve upgraded your tires to a different size and noticed your speedometer is no longer accurate? You’re not alone. Changing tire sizes is a common modification, especially for off-road enthusiasts looking for better performance or a more aggressive look. However, this modification can throw off your speedometer and odometer readings, leading to inaccurate speed displays and mileage tracking. This article will guide you through how to easily recalibrate your speedometer using an OBD2 scanner and a smartphone app, ensuring accuracy and avoiding potential issues down the road.
When I recently swapped out my stock tires for all-terrain tires on my Jeep Patriot, I encountered this very issue. Moving from 235/55/r17 tires to 225/65/r17, while a sensible choice to avoid rubbing, resulted in my speedometer underreporting my actual speed. Online calculators confirmed my suspicions: at a displayed 60 mph, I was actually traveling closer to 64 mph. This discrepancy isn’t just about knowing your true speed; it also affects your odometer’s accuracy, albeit slightly.
An example of a Veepeak Bluetooth OBD2 scanner used for vehicle diagnostics and adjustments.
Luckily, there’s a straightforward DIY solution to correct your speedometer and odometer after a tire size change. Forget expensive trips to the dealership; you can achieve accurate readings yourself with a few affordable tools.
Tools You’ll Need for Speedometer Recalibration
To get your speedometer back on track, you will need the following:
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OBDII Scanner: This Bluetooth device plugs into your car’s OBDII port and communicates with your smartphone. There are many OBDII scanners available online, with Amazon being a popular source. For this guide, and based on recommendations, here are a couple of options:
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Veepeak Bluetooth OBD2 Scanner: https://www.amazon.com/Veepeak-Bluetooth-Diagnostic-Supports-Vehicles/dp/B076XVQMVS/
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Vgate Bluetooth OBD Scanner: https://www.amazon.com/Vgate-Bluetooth-Scanner-Android-Compatible/dp/B06XGB4873
These scanners are reasonably priced and well-regarded in the car enthusiast community.
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Smartphone & “JScan” App: You’ll need an Android or iPhone to run the JScan app. JScan is a powerful application specifically designed for Jeep, Chrysler, Dodge, and Ram vehicles, allowing you to access and modify various vehicle settings, including tire size. You will need to purchase a license within the app, which costs around $20, but this is a one-time fee and grants you access to a wide range of customization options.
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Tape Measure: A simple tape measure is needed to get an accurate measurement of your actual tire diameter.
Step-by-Step Guide to Recalibrating Your Speedometer with JScan
Once you have gathered all the necessary tools, follow these steps to recalibrate your speedometer:
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Connect the OBDII Scanner: Locate your vehicle’s OBDII port. In most vehicles, it’s typically found under the dashboard on the driver’s side. Plug your OBDII scanner into this port.
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Install and Set Up JScan App: Download and install the JScan app on your smartphone. Open the app and follow the initial setup prompts. You will be asked to select your vehicle’s make and model. Eventually, you will be prompted to purchase a license. Complete this purchase to unlock the app’s full functionality, including tire size adjustments.
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Pair OBDII Scanner via Bluetooth: Enable Bluetooth on your smartphone and pair it with your OBDII scanner through the JScan app. This process may vary slightly depending on your scanner and phone, but the JScan app provides clear instructions. You might need to try connecting a few times for it to establish a stable link. Sometimes, having the engine running can help with the connection process.
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Measure Your Tire Diameter: While online tire size calculators like Tire Size Converter can give you a theoretical tire diameter, it’s crucial to measure your tires directly. Using your tape measure, measure the diameter of your tire from the top to the bottom while it is mounted on your vehicle and properly inflated. This actual measurement can differ slightly from the calculated value.
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Enter Tire Size in JScan: In the JScan app, navigate to the section that allows you to adjust vehicle settings or parameters. Look for an option related to “tire size,” “wheel size,” or “speedometer calibration.” The app might display your current (incorrect) tire diameter setting. Enter the actual tire diameter you measured in the previous step. JScan usually requires the diameter in inches.
For example, JScan initially displayed my stock tire diameter as 26.5 inches. While TireSize.com suggested my new 225/65/r17 tires should be around 28.5 inches, my actual measurement was closer to 27 inches. Entering this measured value in JScan corrected my speedometer.
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Apply and Wait: After entering the new tire diameter, confirm the changes within the JScan app. The app will communicate with your vehicle’s computer to update the tire size setting. This process may take a minute or two, so be patient and do not close the app or disconnect the OBDII scanner during this time.
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Verify Speedometer Accuracy: Once the process is complete, test your speedometer’s accuracy. Use a GPS-based speedometer app on your phone (like Google Maps or “Car Scanner ELM OBD2,” which is often recommended with Veepeak scanners) to compare your actual speed with your speedometer reading at various speeds. Ideally, they should now match closely.
The Vgate Bluetooth OBD2 scanner, a budget-friendly option for vehicle diagnostics and adjustments.
Important Considerations and Cautions
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Connection Issues: As noted earlier, establishing a Bluetooth connection between your OBDII scanner and phone can sometimes be finicky. If you encounter connection problems, try restarting the app, disconnecting and reconnecting the scanner, or ensuring your vehicle’s engine is running.
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Wait for Changes to Process: When submitting tire size changes in JScan, allow sufficient time for the app to process the request. Avoid interrupting the process to prevent potential issues.
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Software Glitches: While rare, software glitches can occur. In my experience, I briefly encountered an ABS warning light after making changes with JScan. However, it disappeared after disconnecting the scanner and hasn’t returned. Be mindful of such anomalies.
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Do Not Change Unknown Settings: JScan offers a wide array of vehicle settings. It’s crucial to only modify settings you understand. Incorrectly changing other parameters could lead to unintended consequences or vehicle malfunctions. Stick to tire size calibration unless you are confident in adjusting other settings.
Conclusion
Recalibrating your speedometer after a tire size change using an OBD2 scanner and the JScan app is a straightforward and cost-effective DIY task. It ensures accurate speed and mileage readings, contributing to safer driving and proper vehicle maintenance. By following these steps and exercising caution, you can easily correct your speedometer yourself, saving time and money compared to dealership visits. So, equip yourself with an OBD2 scanner and the JScan app, and take control of your vehicle’s accuracy today!
