The instruments interpret the number of turns of the cable and provide information in numerical form in kilometers (miles) per hour and show the total distance in kilometers (miles). If the transmission is functioning properly, the accuracy of the readings depends on the condition and diameter of the tires. Badly worn (too small) or too large tires can affect the speedometer reading. Replacing tires with tires of a deliberately large diameter (such as large snow tires) can also distort the speedometer readings.
In general, manufacturers agree that a 10% error in readings is acceptable due to wear and tear and other factors. If you plan to perform such a check, always use a section of road marked with mileage posts. Never compare readings from another car: its error may add up to yours, and you will draw the wrong conclusion. The speedometer cable may become dry and loosen the loop. As it rotates, the cable will make a rustling or light tapping sound, which may mislead the owner into suspecting a fault in the engine. If you hear such a noise, carefully watch the needle in the speed range where the noise is heard. In the event of a fault, the needle will jump or deflect with each beat of the cable. The amplitude of the needle's oscillations may be very small, and to notice it, an assistant is needed, sitting behind the driver and continuously watching the speedometer, while the driver is focused on driving.
The slightest difficulty in turning the speedometer cable can cause unpredictable changes in the cruise control system. If the cruise control system vibrates or shows signs of slow acceleration, first check the speedometer cable.
Some cables do not connect directly to the speedometer assembly, but rather to a pulse generator, the pulse current from which is used to determine speed and total distance in miles. In addition, electrical signals that correlate with vehicle speed can be used by the fuel injection control unit, cruise control unit, and other units.
(The article was taken from the website HondaBook.ru)