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How to Reprogram a Computer When Changing Tire Size

by Rita Wood

When you put different size tires on your vehicle, your speedometer will no longer read the correct speed. Larger tires cause the speedometer to show slower speeds than reality, and smaller tires show faster speeds. You can reprogram a vehicle when changing tire size using an on-board diagnostics system, or OBDII. OBDII systems are in most cars and light trucks, and were introduced in the mid 1990s. This diagnostics system monitors parts of the chassis, accessory devices, as well as tire size. You can reprogram your vehicle using an OBDII programmer such as Diablo Sport or SCT.

Step 1

Look under the driver's side dashboard and find the OBDII port. It is black and approximately 2 inches long. It is mounted below the steering column in most vehicles. Plug the OBDII programmer into the OBDII port.

Step 2

Turn your car to the Auxiliary position using your keys to turn on the car's computer and the OBDII programmer.

Step 3

Select "Customize" on the OBDII programmer and enter the new tire size. For example, if you purchased 25575R15 tires enter that into the programmer. When you are finished, click "Update" to reprogram the tire size.

Turn the key to the "Off" position and remove them. Disconnect the OBDII programmer from the OBDII port.

Items you will need

  • OBDII programmer

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