The calibration of a speedometer is based on the torque that the magnetic field creates and takes into account the ratios of the gears in the drive cable, the final drive ratio in the differential and the diameter of the tires. A large change in the size of a car's tires is one of the most common ways a speedometer's accuracy is compromised because it affects how far the car goes with each revolution of the tires.
The manufacturer of the car calibrates the speedometer to correspond with the factory-installed ring, pinion ratio and tire size. Vehicle manufacturers typically calibrate speedometers to display a speed faster than the speed at which the car is actually travelling. This is done for safety and liability reasons so that drivers do not go faster than they think they are going and vehicle manufacturers are not held liable in such occurrences.
Drivers that make significant changes to the car should recalibrate the speedometer by manipulating the hairspring, the permanent magnet or both. To change the magnetic field, the car's owner needs to use a strong electromagnet that is capable of adjusting the power of the permanent magnet in the speedometer until the input from the rotating drive cable matches the needle.