Upgrading your vehicle with new tires can significantly enhance its performance and appearance. However, changing to a different tire size than your original equipment manufacturer (OEM) specification can throw off your speedometer and odometer readings. This inaccuracy occurs because your vehicle’s computer calculates speed based on the rotational speed of the tires, which is directly related to the tire’s diameter. Larger or smaller tires will cover a different distance per rotation compared to the stock tires, leading to speedometer misreadings. Fortunately, with the help of an OBD2 scanner and a compatible smartphone app, you can easily recalibrate your speedometer to ensure accurate readings after a tire size change.
Understanding Tire Size and Speedometer Accuracy
Before diving into the recalibration process, it’s crucial to understand how tire size affects your speedometer. Tire sizes are typically represented by a combination of numbers and letters, such as 225/65R17. This code breaks down as follows:
- 225: The width of the tire in millimeters.
- 65: The aspect ratio, representing the sidewall height as a percentage of the width.
- R17: Indicates a radial tire construction and a wheel diameter of 17 inches.
When you change tire sizes, especially the aspect ratio or wheel diameter, you alter the overall diameter of the tire. A larger diameter tire will cover more ground per revolution, meaning your speedometer will read lower than your actual speed. Conversely, a smaller diameter tire will result in a speedometer reading higher than your actual speed. This discrepancy not only affects your speedometer but also your odometer, potentially impacting mileage tracking and fuel efficiency calculations.
Alt text: Detailed view of tire sidewall markings showing size specifications, aspect ratio, and diameter for understanding tire dimensions.
Tools You’ll Need for Speedometer Recalibration
Recalibrating your speedometer after a tire change is a straightforward process that requires minimal tools. Here’s what you’ll need:
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OBD2 Scanner: This is the key tool for communicating with your vehicle’s computer system. OBD2 (On-Board Diagnostics II) scanners plug into your car’s OBD2 port, typically located under the dashboard. While there are many brands available, popular options include Veepeak and Vgate, as mentioned in the original article, and BlueDriver. BlueDriver OBD2 scanners are known for their user-friendly interface and compatibility with various vehicle makes and models, making them a reliable choice for this task.
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Smartphone or Tablet with Compatible App: You’ll need a smartphone or tablet (Android or iOS) to run a compatible OBD2 app. Apps like JScan (popular among Jeep owners), Car Scanner ELM OBD2, and manufacturer-specific apps (like the BlueDriver app for BlueDriver scanners) allow you to access vehicle settings and perform calibrations.
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Tape Measure: A simple tape measure is needed to accurately measure the actual height of your new tires, as real-world dimensions can sometimes vary slightly from the manufacturer’s specifications.
Alt text: Veepeak OBD2 Bluetooth Scanner connected to a vehicle’s OBD2 port with a smartphone showing a car diagnostic application interface for vehicle communication.
Step-by-Step Guide to Recalibrating Your Speedometer Using an OBD2 Scanner
Once you have the necessary tools, follow these steps to recalibrate your speedometer for your new tire size:
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Connect the OBD2 Scanner: Locate the OBD2 port in your vehicle (refer to your car’s manual if you’re unsure of its location). Plug your OBD2 scanner securely into the port.
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Install and Connect the OBD2 App: Download and install the compatible OBD2 app on your smartphone or tablet. For BlueDriver scanners, this would be the BlueDriver app. Launch the app and follow the instructions to connect to your OBD2 scanner via Bluetooth. You may need to pair your device with the scanner through your phone’s Bluetooth settings first.
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Measure Your Tire Diameter: Use a tape measure to determine the actual diameter of your newly installed tires. Measure from the very top to the very bottom of the tire while it’s mounted on your vehicle. While online tire size calculators can provide estimated diameters, a physical measurement ensures accuracy.
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Access Tire Size Calibration Settings in the App: Within the OBD2 app, navigate to the vehicle settings or configuration menu. Look for options related to “tire size,” “wheel size,” or “speedometer calibration.” The exact menu names and locations will vary depending on the app and your vehicle’s make and model. For apps like JScan, these settings are typically found within the “Adaptations” or “Service Procedures” sections. With BlueDriver, you would navigate through the app’s menus to find vehicle-specific adjustments.
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Enter the New Tire Diameter: The app will likely display the current tire size setting (often in inches). Input the measured diameter of your new tires into the designated field. Some apps may allow you to enter the tire size code directly, while others require the diameter in inches.
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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 few minutes, and it’s crucial to remain patient and not interrupt the process. Ensure your vehicle’s ignition is in the “ON” position (engine may or may not need to be running, depending on the app’s instructions).
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Verify Speedometer Accuracy: Once the calibration process is complete, take your vehicle for a test drive. Use a GPS-based speedometer app on your smartphone or a GPS device to compare your actual speed with your vehicle’s speedometer reading. Ideally, they should now be closely aligned. You can also use roadside speed signs as a reference point.
Important Considerations and Cautions
- App and Scanner Compatibility: Always ensure that your chosen OBD2 scanner and app are compatible with your vehicle’s make, model, and year. Check the manufacturer’s compatibility lists before purchasing.
- Software Glitches: As mentioned in the original article, occasional glitches or connection issues can occur with OBD2 scanners and apps. If you encounter problems, try restarting the app, reconnecting the scanner, or consulting the app’s troubleshooting resources.
- Measure Accurately: Precise tire diameter measurement is critical for accurate speedometer calibration. Take your time and measure carefully.
- Proceed with Caution: OBD2 apps provide access to various vehicle settings. Only change settings you fully understand, and strictly follow the app’s instructions for tire size calibration to avoid unintended consequences or system errors. If you are unsure about any step, consult your vehicle’s repair manual or seek professional assistance.
Conclusion
Recalibrating your speedometer after a tire size change using an OBD2 scanner is a relatively simple and cost-effective DIY task. By following these steps and using the right tools, you can ensure your speedometer and odometer provide accurate readings, maintaining safety and proper vehicle operation. Whether you choose a BlueDriver OBD2 scanner or another compatible device, taking control of your vehicle’s calibration empowers you to customize your ride with confidence.
