Devices interpret the number of turns of the cable and give 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 reading depends on the condition and diameter of the tires. Heavily worn (too small) or tires that are too large may interfere with the speedometer reading. Replacing tires with tires of obviously large diameter (type of big snow tires) can also distort the speedometer readings.
In general, manufacturers have agreed that a reading error of 10% is considered acceptable due to wear and tear and other causes. If you plan to do this check, always use the section of road marked with mileposts for this. Never use the instrument readings of another car for comparison: its error may add up to yours and you will draw the wrong conclusion. The speedometer cable can become dry and loosen the loop. Due to its rotation, the cable will make a rustling or light tapping, which can mislead the owner, who will suspect a malfunction in the engine. If you hear such a noise, carefully observe the arrow in the speed range where the noise is heard. In the event of a malfunction, the arrow will bounce or deviate with each beating of the cable. The oscillation amplitude of the needle can be very small, and to notice it, you need an assistant sitting behind the driver and continuously watching the speedometer, while the driver is focused on driving.
The slightest difficulty in rotating the speedometer cable can cause unpredictable changes in the system for setting and maintaining optimal speed (cruise control). If the cruise control system exhibits vibration or signs of slow acceleration, check the speedometer cable first.
Some cables do not connect directly to the speedometer assembly, but more often to an impulse generator, the impulse current from which is used to determine speed and total mileage. In addition, electrical signals correlating with vehicle speed can be used by the fuel injection control unit, the unit for setting and maintaining the optimum speed (cruise control) and other blocks.