What does a speedometer do?
Your speedometer monitors the speed of the car. In the past this would be done through a cable, but nowadays the speedometer is mainly an electric part that relies on information from sensors to calculate the speed.
How do you know there is something wrong with the speedometer?
The Speedometer is usually the most prominent part of a car's dashboard. It is rare for the speedometer to fail but if it does there are usuallly a number of potential reasons for this. You may not notice innacuracies in the speed, especially if they are only a few MPH out. It's easier to spot if the system is completely inactive, or weak and erratic.
When should you consider a speedometer replacement?
The speedometer is a vital tool for drivers, so should last the lifetime of the vehicle. That being said it can breakdown in a variety of ways.
What happens during a speedometer replacement?
A mechanic will come to replace the speedometer, and assess the situation on the vehicle to see if there are any other problems. The mechanic will then replace the speedometer and advise if there are further parts to replace. They will then test the system and adjust as neccessary.