Speedometer Repair: How to Recalibrate a Speedometer

A speedometer on a car.

The standard speedometer gets calibrated at the factory, however, it may eventually become necessary to undertake speedometer repair if it doesn't seem to be measuring speed and mileage accurately. Using tire circumference as well as height, engineers are able to ensure the speedometer operates properly. If the axle ratio or tire size or both are changed, you need to invest in speedometer repair to recalibrate the speedometer.

Step 1 - Understand the Connection

Your vehicle speedometer is connected with the transmission through a speedo cable. Within the transmission are located two gears, namely the driving and the driven gear. The driving gear is also called the trans gear and is situated over the output while the driven gear is called speedo gear and is directly attached to this speedo cable. When undertaking speedometer repair, you need to replace this speedo gear. When you change both or one of the gears, you end up changing the cable speed.

Step 2 - Change the Speedo Gear

You can change the driven gear or the speedo by removing the speedo cable away from the transmission. After this, you need to remove the lock bolt and then slide out the gear housing from inside. This should bring you closer to the process of speedometer repair. Some of the gears can be as much as 3/4 inches in diameter while others could range between 1/2 and 1 inch in diameter. You need to measure the precise size needed because mixing up the two diameters would result in a mismatch. You can replace the other driving gear only after removing the tail shaft of the transmission.

Step 3 - Choose Gear Size

Now you know how to ensure speedometer repair through recalibration, but how can you choose the appropriate gear size? You can select the appropriate gear as per the tire diameter and axle ratio combinations. Irrespective of the vehicle you own, you can estimate the appropriate gear with the following formula. Start by multiplying rear axle with 20.2 and then divide this number by tire diameter. You need to round this number off to the nearest calibration suited for the speedometer repair.

Step 4 - Check Readings

Once you have selected the appropriate gears, check the readings in the odometer over a specific distance. If you find any errors, these will typically originate from inaccurate tire diameter calculation. Make sure to re-measure the tire diameter. If you find correct odometer readings but discrepancies in the speedometer readings, you would need to replace the speedometer because it means there is an issue with the speedometer.

Step 5 - Understand Tire Diameter

The diameter of the tire will have an impact on the speed shown on the speedometer and therefore impact the speedometer repair as well. Compared to smaller diameter tires, the larger tires will travel greater distances in the same time. For example, a tire with a 20-inch diameter will include 62.8 inches in a single revolution while a bigger tire with a 30-inch diameter will cover almost 94.2 inches of ground. You can recalibrate the speedometer and finish the speedometer repair by changing the hairspring. The drive cable is attached to a magnet via spiral gear and this magnet is inside a metal piece in the shape of a cup called speedcup. The connection between this speedcup and the needle is via the hairspring. Y