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A Guide to Speedometer Calibration

A Guide to Speedometer Calibration

Speedometer calibration is a necessary part of car ownership and important to keep a car in top condition and ensure that driers do not receive any unwanted tickets for speeding without being aware of it. The speedometer of car is used to give an approximation of the speed that the car is going. Contrary to popular belief, the speedometer does not record the actual speed of a car but rather the number of driveshaft revolutions per mile.

Speedometer calibration is the process by which a car owner adjusts the speedometer on their car to accurately reflect the speed a car is traveling. Speedometer calibration initially occurs at the car manufacturing factory to show the accurate speed of a car within about 1-5%. However, if a car is ever adjusted in the future from the factory specifications, it most likely will require a speedometer calibration again. Different parts of a car that affect the speedometer include the gears in the drive cable, tire size and the final drive ratio in the differentials. A differential is the mechanism in cars that sends engine power to the tires, slows down this power accordingly and sends different speeds to different tires, hence its name, differentials. If adjustments are made to any of these parts of a car speedometer calibration is required.

A local mechanic or car dealership that services the brand of your vehicle will most likely be able to assist you in speedometer calibration. Alternatively, if you would first like to determine if your speedometer is recording speed precisely there are a couple of simple tests to run. First, you can take your car out to a stretch of highway with mile markers, set your car’s cruise control to 60 miles per hour and time how long it takes you to drive one mile. It should take 60 seconds, however if it is recorded as faster or slower this indicates you may need your speedometer calibrated. Second, you can find a state highway patrol or city police radar detector that is set on the side of the road, typically in school zones or residential areas, and test your speedometer against what the radar is recording.

Once you have determined that you need to calibrate your speedometer and that you would like to calibrate it yourself, start by removing the speedometer from the car using an 11 millimeter wrench and needle nose pliers. The pliers will remove the retaining clip and then you can remove the gear from the speedometer. Be sure to have a washcloth on hand wipe away any transmission fluid. Install a new gear from your auto dealer (pre-order this before removing your current gear) and reassemble the speedometer back into the car. Be sure to test the new speedometer. Your auto dealer will know which gear you need to accommodate the difference in speed that your speedometer is recording.

An alternative route is to adjust the magnet field within the speedometer using a large, powerful electro magnet. This electromagnet will be able to adjust the permanent magnet and adjust the needle accordingly.

Speedometer calibration is a good practice for all car owners to keep in to ensure that their speedometer is reflecting the most accurate speed possible. 

Photo courtesy of smemon87.

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