How to Adjust a Digital Speedometer
An accurate speedometer can make the difference between a smooth drive and a ticket. A reading that is off by only a few miles per hour in the wrong direction can easily put you on the wrong side of the speed limit. A speedometer is easily affected by minor adjustments to the car itself. One of the most common changes that can drastically change your speedometer's readout is a change in tire size.
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Instructions
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Check your speedometer for accuracy. You can find accuracy algorithms available online. These websites will ask you to input simple numbers, specifically the time it took you to travel one mile at a set speed. For example, if you traveled one mile in one minute and registered a speed of 50 mph, then your speedometer is off by 20 percent. The correct reading should be 60 mph. This will give you the amount of correction that is needed.
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Purchase a speedometer adjustment device. This device will plug into your car's OBD2 reader. The device will allow the user to change various settings and readouts within the car. These will run in price from $100 on the low end to well over $1000 on the high end. To make sure that you purchase a quality device, be sure to read reviews and talk to other owners.
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Follow the instructions provided with the OBD2 speedometer adjustment device. These will vary from device to device. However, in general, there will be a menu that lists car makes and models. Select the needed model; the device will have a list of options that can be changed. This includes settings such as odometer and engine codes.
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Retest the speedometer. Take the car back out onto the road. Go one mile for a full minute at a sustained speed. Run the numbers through the same website you used for your original testing. If the speedometer is correctly adjusted, then you are finished. If, however, there is still a discrepancy, then repeat the process.
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Tips & Warnings
Many modern speedometers have automatic calibrators built in. If you see your speedometer jump to the middle position when you start your car, then it is automatically calibrating.
References
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