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The traditional speedometer is a mechanical type. A typical mechanical odometer connects a flexible shaft. There is a wire cable in the flexible shaft. The other end of the flexible shaft is connected to a certain gear in the transmission. The gear rotates to rotate the wire cable. Driving a magnet in the odometer cover circle, the cover ring and the pointer are connected and the pointer is set to zero through the hairspring, and the speed of the magnet rotation speed changes the size of the magnetic force line, and the balanced broken pointer is driven.
This speedometer is simple and practical and is widely used in small and large cars. However, with the development of electronic technology, many automobile meters now use electronic speedometers. One common method is to obtain signals from the speed sensors on the transmissions, and to deflect or display numbers by changing the pulse frequency. 
The odometer is a kind of digital instrument. It meshes with the worm on the propeller shaft of the speedometer through the drive gear of the counter drum wheel to rotate the counter drum. Its characteristic is that the upper drum wheel rotates one complete revolution, and the next drum revolution 1/10 laps. Just like the speedometer, the current odometer also has an electronic odometer, which obtains the mileage signal from the speed sensor. Mileage figures accumulated by electronic odometers are stored in non-volatile memory and can be stored in the absence of electricity.
Another more conspicuous instrument is the tachometer. In domestic cars, the tachometer was not usually used before. However, in recent decades, all types of automobiles have risen and installed tachometers, and some manufacturers have also used it as the configuration content of car grades. The tachometer unit is 1/min*1000, which shows how many thousand revolutions per minute the engine rotates. Tachometer can intuitively display the engine speed in various operating conditions, the driver can always know the engine operating conditions, with the transmission gear and throttle position, so that it maintains the best working condition, to reduce fuel consumption, extend engine life benefit.
The tachometer is usually set inside the dashboard and placed symmetrically with the speedometer. The tachometer works according to the magnetic principle. It receives the pulse signal generated when the primary current in the ignition coil is interrupted, and converts this signal into a displayable speed value. The faster the engine speed, the greater the number of pulses generated by the ignition coil and the greater the speed value displayed on the chart.
Electronic tachometer, there are pointer and liquid crystal digital display, the table has a digital integrated circuit, it will drive the voltage pulse sent over the ignition coil after calculation to drive the pointer to move or digital display. There is also a tachometer that takes the pulse signal from the generator to the tachometer circuit and displays the speed value. However, due to factors such as generator belt slippage, the value is not very accurate.
The oil pressure gauges, water temperature gauges and fuel gauges on the dashboard are directly related to the engine work. Each of them has a corresponding sensor to reflect the information of the monitoring object on the instrument.
How car speedometers, tachometers, and sensors work with each other