Hello..
Today, I want to learn about speedometer, one of important part at our car. We all know, that speedometer show about how fast your car run but there's more about it that we don't know… So, come on and read about it :-

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s we know speedometer work to indicate the speed of our car in miles per hour, km per hour or both (depend on your car ). It's a device that uses a needle to point specific number based on the speed of the vehicle. It's an expensive device in beginning and only available as an option until 1910 when automaker began to include it as the standard equipment in all vehicle. The first speedometer supplier was Otto Schulze Autometer(OSA). The fist OSA speedometer was built in 1923 and the basic design didn't change for 60 years.
There's are 2 types of speedometer :-
a. Electronic speedometer
- First appear in 1993.
b. Mechanical speedometer
- The current and mostly used speedometer.

This Is Speedometer Vol 1 & 2
Learn more

Back to the history of Speedometer, the invention of mechanical speedometer (or called eddy-current) allowed drivers to check their speed and they can adjust their travelling based on the speedometer reading. At the same time, many countries make a law for speed limits.
The basic eddy-current speedometer parts is :-

1. Piston engines use energy from a burning fuel-air mixture to move a piston up and down in a cylinder.
2. This reciprocating motion of the pistons is converted into rotary motion by a crankshaft.
3. The crankshaft turns a flywheel.
4. The transmission transmits power from the flywheel and directs it, through a driveshaft, to the wheels.
5. The transmission has different gears — or speeds — to control how fast the wheels turn.
6. As the wheels turn, they cause the car to move.

The measurement of car speed used the rotational speed of either wheels or the transmission and the information will be send to some sort of gauge. The measurement of the rotational speed will be transmitted through drive cable.
That's for eddy-current/mechanical speedometer. For electronic speedometer, the data will receives from a vehicle speed sensor (VSS). It is mounted to the transmission output shaft or to the crankshadt and consists of a toothed metal disk and a stationary detector that covers a magnetic coil.

SPEEDOMETER ACCURACY
No speedometer can be 100 percent accurate. In fact, most manufacturers build speedometers so they fall within a fairly narrow tolerance range, no more than 1 percent to 5 percent too slow or too fast. As long as a car is maintained at factory specs, its speedometer should continue to register vehicle speed within this range. But, if a car is modified, its speedometer may need to be recalibrated.
Changing tire size is one of the most common things car owners do that can affect speedometer accuracy. That's because larger tires cover more ground in one complete revolution. Consider the example below.

Reference :- AUto Howstuffworks

Maybe it is a bit technical but some of the history and how it works can be understand.

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