Proper speedometer calibration is important because incorrect readings can result in a ticket if a driver is moving to fast and it can annoy other drivers if it is traveling to slow.
A speedometer measures the torque of the magnetic field that is created by a moving vehicle. The speedometer calibration includes several factors including the size of the wheels and ratios of all the gears that are involved in moving the vehicle.
Changing the size of the wheels of a vehicle is a common reason why it may need to be recalibrated. Larger tires can make a speedometer read a slower speed because each revolution makes the tire move further, while smaller tires can result in a faster reading than what it actually is. This can happen when there has been lots of wear on the tire and the diameter has been reduced.
American and Japanese manufacturers both adhere to a plus or minus four percent rule for accuracy. One sure way to test for accuracy is to drave past a speed radar that many highway patrols place on the side of roads. A driver can determine any inaccuracies by driving past one of them. Another method is to download a smartphone application or to get a GPS device. However, these methods do not take into account changes in elevation.
There are two types of speedometers mechanical and electronic. Take for example the size of the drives gears. With fewer teeth, the gears are slowed down, and with more teeth, the gear speeds up and this can throw off the readings. Mechanical speedometers have a cable attached to the transmission. If you were to change the size of the tires, you would have to calculate the size of the drive and contact a manufacturer for replacement.
If the tire size is changed on a vehicle that has an electric speedometer, then there is much less work involved. There are three ways to get it done. The car can be driven exactly one mile, adjust to a reference point, or use the car’s sensor that adjusts to point per mile.