Every time you look at the dashboard while you're moving, you see speedThis is one of the most important parameters for the driver. It is on the basis of these figures that we make decisions about overtaking, braking and compliance with traffic rules to avoid fines. However, few people think about the complex path that a signal takes from wheel to arrow or a number on the screen.
Modern cars use sophisticated electronic systems to calculate current speeds, relying on data from various sensors. This data is processed by the control unit and displayed on the display with a certain delay and, most importantly, with the error laid by the manufacturer. Understanding the principles of this node will help you feel the car on the road better.
In this article, we will discuss in detail why the speedometer almost always “lies” in the big way, how the size of the rubber affects the accuracy of readings and how mechanical actuators differ from electronic ones. You'll find out why. GPS-navigator It often shows speeds lower than the dashboard and whether it is worth trusting.
Principles of speed measurement: from cable to electronics
Historically, the first cars were equipped with mechanical speedometers, which were driven by a flexible shaft connected to the gearbox. The rotation of the shaft was transmitted directly to the mechanism of the device, where the magnetic field caused the arrow to deviate to a certain angle. It was a simple but reliable system, which, however, had high wear and depended on a lot of rubbing parts.
With the development of the automotive industry, electronic speed sensors came to replace the mechanics, which are installed directly on the output shaft of the transmission or even on the hubs of the wheels. Hall's sensor. or inductive sensor generates electrical impulses, the frequency of which directly depends on the speed of rotation of the shaft. These pulses are sent to the control unit, which counts them in kilometers per hour.
In modern cars, speed data is often taken from the system. ABSEach wheel has its own sensor. This allows the system not only to show speed, but also to adjust the engine and brakes in real time. This scheme provides high accuracy and reliability, since failure of one sensor does not always lead to complete loss of information.
⚠️ Note: When replacing the mechanical cable speedometer, it is extremely important to use the lubricant recommended by the manufacturer. The use of thick lubricants in winter can lead to their freezing and breaking the cable.
Electronic systems also allow information to be displayed not only on an analog scale, but also on a digital display, where readings can be duplicated in projections on the windshield. This creates redundancy of information, which increases driving safety, allowing the driver not to be distracted from the road to find the arrow.
Why Speedometers Always Show More: Law and Physics
Many drivers have noticed that when driving on the highway with a permitted speed of 110 km / h, the navigator shows only 100-102 km / h. This is not a breakdown or accident, but a legally enshrined norm in many countries, including Russia and the EU. UNECE Regulations The speedometer has no right to show a speed less than the real one, but can exceed it.
The error formula usually looks like this: the shown speed should not be less than the real one, but can exceed it by no more than 10% plus 6 km / h. This is done so that the driver does not accidentally violate the speed limit due to inaccuracy of the device or change in the diameter of the wheels. The manufacturer insures himself and the owner against fines, artificially inflating the readings.
- 🚗 At a speed of 60 km / h, the real figure can be about 52-55 km / h.
- 🛣️ On the track at 120 km / h on the instrument, the actual speed is often 110-113 km / h.
- 📉 The higher the speed, the greater the absolute error in kilometers.
- 🔧 Electronic speedometers are usually more accurate than mechanical ones, but also have a margin.
This approach also takes into account the wear of tires. When the tread is worn, the diameter of the wheel decreases, and in one revolution it travels a shorter distance. If the calibration was perfect, then on bald rubber, the speedometer would begin to underestimate the readings, which is unacceptable by law. Therefore, the factory "stock" covers both wear and replacement of tires with non-standard ones.
⚠️ Note: In some jurisdictions, the speedometer readings may be used as evidence in court. However, in case of a dispute, priority is always given to data from certified radars or video recorders with GPS.
Effect of tyre size on accuracy of readings
One of the most important factors affecting accuracy speedThe actual outer diameter of the wheel. The car computer only knows the number of revolutions that the wheel has made, and the basic circumference length laid down in the memory of the calibration. If you change the size of the tires, these calculations are confused.
When installing wheels larger than the factory, the real speed of the car becomes higher than the speedometer readings. The wheel makes less revolutions per kilometer of travel, the sensor “thinks” that the car is going slower, and shows understated figures. This can be dangerous as you may unknowingly speed up.
In the opposite situation, when the wheels of a smaller diameter are put, the speedometer begins to “lie” even more in the big direction. The car is going slower than the arrow shows. While it is safe in terms of penalties, it can be annoying if it is necessary to maintain an accurate speed mode in the flow.
Before buying new wheels or tires, use the online tire calculator to check how much the actual speed and clearance of the car will change at the selected size.
For example, replacing the 205/55 R16 bus with the 205/60 R16 will significantly increase the diameter and distort the instrument readings.
Comparison of speedometer and GPS-navigator data
Modern drivers often check the readings of a regular speedometer with the data of a smartphone or built-in navigator. GPS receiver He calculates the speed in a completely different way: he measures the change in the coordinates of an object per unit of time. This eliminates the effects of wheel diameter, slipping or calibration of sensors.
However, GPS navigation has its own characteristics. Satellite speed is updated at a certain frequency (usually 1 time per second), so on sudden accelerations or braking, the numbers can change with a delay. In addition, in tunnels, dense forest or between high-rises, the signal may disappear, and the speed will cease to be displayed.
The standard speedometer reacts instantly, as it is associated with the rotation of the wheels. That is why it is better to look at the dashboard to control sharp accelerations and precise cornering, and to control the average speed on a long stretch, trust the navigator.
| Parameter | The regular speedometer | GPS-navigator |
|---|---|---|
| Principle of work | Wheel rotation/shaft | Change of coordinates |
| Tyre dependence | Tall. | Absent. |
| Signal delay | Minimum | 1-2 seconds |
| Tunnel work | It's working. | It's not working. |
Types of speedometers: analog and digital
Despite digitalization, analogue The arrow remains the standard for most cars. The human brain reads the position of the arrow on the scale faster than a specific number, which allows the driver to control the speed with peripheral vision without leaving the road. The mechanism of such devices is frequent, where the arrow moves in jerks, imperceptible to the eye.
Digital indicators that display speed in the form of numbers are becoming more popular, especially in electric cars and sports cars. They provide high read accuracy, but require focusing the eye. Often, manufacturers combine these approaches, leaving a large analog scale and inputting the exact figure into the center.
- 👁️ Analog scales are better perceived by lateral vision.
- 🔢 Digital scoreboards more accurately show the current value.
- 🎨 Virtual dashboards allow you to change the scale.
- 📉 At high speed, digital values can flash too fast.
Virtual dashboards are essentially high-resolution screens that mimic any type of speedometer. You can switch between Comfort, Sport or Navigation modes, changing the priority of the displayed information. This gives flexibility, but requires getting used to different interfaces.
What is a virtual scale?
A virtual scale is a software simulation of a speedometer on an LCD screen. It has no physical moving parts, and the arrow and numbers are drawn by the GPU. This allows you to change the design, add animations and output any additional information directly to the speed scale.
Failures and diagnostics of speed sensors
If speed Behaving inadequately - the arrow jumps, lies at zero or goes off the scale - this is a signal of malfunction. In mechanical systems, the cable is most often torn or the gears of the drive wear out. In electronic systems, problems lie in the sensor, wiring, or control unit itself.
One of the signs of a faulty speed sensor on modern cars is incorrect operation of idling or jerking when driving. The electronic engine control unit (ECU) uses speed data to correct the fuel-air mixture. If the signal is lost, the car may become dead when braking.
Diagnosis usually begins with a computer scan. Errors in the ABS system or transmission will indicate a problem with a specific sensor. The integrity of the wiring and connectors can also be checked, as contact oxidation is a common cause of failures, especially after winter operation.
☑️ Speedometer diagnostics
⚠️ Note: If the ABS or Check Engine lights up on the dashboard at the same time as the speedometer fails, the vehicle may be restricted to safety. It is recommended to contact the service.
Legal aspects and calibration
The calibration of the speedometer is especially relevant for commercial vehicles and taxis. In some countries, there are strict requirements for the accuracy of the appliances used for charging or controlling the work schedule of drivers. For passenger cars, mandatory periodic verification of the speedometer is usually not required, unless the design of the car changes.
However, if you install non-standard sized wheels, you are legally making changes to the vehicle design, which may require changes to the documents. In this case, the speedometer readings become “illegal” from the point of view of factory calibration, although in practice this is rarely checked.
In case of disputes with speed cameras, the data of the speedometer of the car is rarely taken into account as proof of innocence. The main evidence is considered to be the testimony of a certified measuring complex. Therefore, to rely on the fact that “speedometer showed less” when challenging fines is pointless.
Factory error speedometer is not a marriage, but a requirement of safety. Do not try to programmatically "correct" the readings if you do not change the diameter of the wheels, as this can lead to accidental traffic violations.
Why is electric cars faster than the speedometer?
In electric vehicles, speed is often measured directly by the speed of the electric motor rotor, which is rigidly connected to the wheels through a gearbox. The lack of transmission slip and high accuracy of electronic sensors allow for a minimum error, although manufacturers still leave a small margin.
Can I refashion the speedometer under the big wheels?
Yes, in many modern cars, through a diagnostic connector, you can change the calibration coefficient of the size of the tires in the control unit. This allows you to correctly display speed and mileage after installing wheels of non-standard diameter. However, such work should be carried out by qualified specialists.
Does the pressure in the tires affect the speed?
Theoretically, yes, since the flat tire has a smaller rolling radius. However, in practice, the difference in diameter between the inflated and deflated tires is a fraction of a percent, which is imperceptible against the background of the standard factory error of the speedometer in 5-10%.
What if the speedometer shows 0 when moving?
It's a critical malfunction. Most likely, the speed sensor failed or the wiring to it broke. You can drive, but with caution, since speed limiters do not work, automatic transmission and the stability system may not work correctly. We need to run a diagnostic.