Personally, it has always been interesting to me how it is possible to measure the exact speed of a car while driving. I read about it a little long ago and realized that it goes something like this. There are several ways to measure speed, and it all comes down to turning the wheels.
In older models, the sensor was on the differential, because for years this method was considered the most reliable for measuring speed. The differential can only be connected to the speedometer by a cable.
In modern cars, speeds are optically measured via sensors from the ABS system and measured by turning the wheels, which is why wheel circumference is very important.
The larger the outer circumference of the tire, the greater the distance the wheel will travel in one full revolution. If the circumference of the wheel is 1 meter - once it turns, it will cross a path of one meter. Circumference 2 meters - with one turn it will cross 2 meters.
The speedometer was calibrated to that range. Therefore, when the system registers that the wheel has turned 100 times while driving, it will take its circumference and multiply it by 100. Take, for example, a tire of dimensions 195/65 R15. It has a circumference of about 2 meters.
When that wheel turns 100 times while driving, it means that it has crossed 200 meters. Then we are interested in what time he crossed those 200 meters. If he crossed them in 10 seconds, it means that he was moving at a speed of 20 meters per second or 72 km / h.
I am aware that this is a complicated explanation, but I still hope that it is a little clearer to you where to find the speed of the car.
Honestly, anything over 140 is panic for me. Whether it’s in the old format or whether it’s in digital.