Compared to the speedometer that may frequently experience mechanical errors or potentially may be deliberately set back, the sat nav speed is more accurate as it works through calculating the time needed to travel from one point to another.
That is the primary reason why the sat nav can provide a more accurate reading. Why do sat navs offer a more precise reading than speedometers?
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How can GPS devices and speedometers measure the speed of your vehicle?
To understand why the speedometer provides somewhat inaccurate readings, you first must understand how GPS devices and speedometers measure speed. The speedometer that your vehicle has may either be mechanical or electronic.
People have been using mechanical speedometers since the 1900s, while the first electronic speedometer appeared in the late 1980s. Many modern vehicles have been using the electronic speedometer so they can gauge and show the vehicle’s speed.
How does a car speedometer work?
Speed involves the measurement of the distance over some time. However, the car speedometer does not precisely measure the speed at which you travel from one point to another. Car speedometers often work through measuring how the car’s wheel, axle, or drive shaft rotates.
After that, they use some basic mathematical computations to have that rotation extrapolated and determine your travel speed. It can be very much the same as the concept of the bicycle’s speedometer.
Despite that, once the diameter of the tyre or wheel gets altered, the extrapolated calculation will no longer be correct. For example, if you put new tyres on your car, the diameter would increase. When that happens, the tyre pressure will increase, and the tyres will experience more tread that can wear down over thousands of miles.
Thus, the speed will be more significant for every revolution of the wheel that the car is travelling farther.
Once the diameter increases, because you are using a different tyre brand with slightly different dimensions, the tyres can get worn out, or the wheels would have less air. The car would travel a shorter distance for every revolution of the wheel, so you will go slower.
The Car Speedometer’s Margin of Error
The wheel diameter’s difference that results from these circumstances may only be a few millimetres. Despite that, if you travel at 30 miles per hour and your car wheels rotate six to seven times each second, it can immediately create a few miles of difference each hour.
When it comes to its application in law, they consider this error margin and how manufacturers calibrate their cars’ speedometers.
The Mechanical Speedometer
With simple construction, mechanical speedometers consist of these parts: the dashboard’s terminal pointer needle, a hairspring, speed cup, drive cable, and magnets. The flexible line that is the drive cable is a group of small helical coil springs tightly wound around the mandrel or central wire.
It is then attached to the vehicle’s transmission. The drive cable connects to the gear wheel through a small pinion gear on the transmission’s output shaft for better precision. The output shaft then connects to the vehicle’s wheels and imparts its rotational speed.
Because of that, it will join the speedometer’s drive cable to the output shaft, which can assess the car’s speed that is relatively accurate. Once the vehicle operates, the output shaft’s rotary motion will get carried over to the drive cable’s central mandrel.
Therefore, the drive cable will rotate at a speed similar to the output shaft. It will rotate at speed the same as the car’s wheels. The drive cable’s other end is then connected to a magnet sitting within the speed cup. The magnet will not touch the speed cup’s interior as a particular gap in the air will be maintained between these two.
It will then impart the drive cable’s rotary motion onto the magnet. Because of that, the magnet will create a rotating magnetic field around it. The rotating magnetic field will then have small eddy currents induced in the speed cup.
These eddy currents will then have a torque that will force the speed cup and eventually the pointer needle to rotate in a direction that is the same as the magnet. If the output shaft rotates faster, the magnetic field will have greater strength, forcing the pointer needle and the speed cup to move farther with the speed dial.
Resist the torque on the speed cup with the hairspring at the pointer held at zero whenever the vehicle does not operate. The pointer needle will stop turning once the torque equals the hairspring’s opposing torque because of the eddy currents that point to the speed on the dial’s correct value.
The Electronic Speedometer
Compared to their mechanical counterparts, electronic speedometers are so much simpler. The vehicle speed sensor or rotational speed sensor is mounted on the transmission’s output shaft. It allows for the transmission of electronic pulses that correspond to the shaft’s rotation speed.
The electronic speedometer is more straightforward than the mechanical speedometer as it only has four parts when it comes to construction. These are the tiny circuits that interpret the readings, the magnetic field sensor, the digital or analogue display unit, and the magnet.
The sensor setup is composed of a circular magnet. It encloses a metal disk that has tiny teeth. Once the shaft spins, the teeth would obstruct the magnetic field that the magnetic field sensor picks up. A computer will then count these pulses and compare them to the total number of times the wheel turns, which would eventually provide information on the car’s speed.
GPS – Global Positioning Satellite
The GPS device provides the easiest possible way to calculate speed. With the use of this tool, you can measure speed through the classic formula that speed is equal to the distance divided by time.
Here, the GPS device will track the car’s location over time and measure how far the vehicle was able to travel. To become precisely aware of what you need to check about sat navs and how these can help your journey become smoother, check out our Comprehensive Guide on the Best Sat Navs.
The value of the travelled distance will then be divided by the time it took your car to travel from one point to another. The computed number will indicate the speed at which the vehicle travels from one point to another. Since GPS devices use the vehicle’s position for the calculation of the speed, they are also referred to as positional speedometers.
Satellite navigation units that may either be integrated with your car or portable can calculate the speed of your vehicle by measuring the actual distance that it travels over time with the use of GPS satellite tracking.
These satellite navigation units repeatedly locate where you are situated precisely on earth through satellites, and they calculate how far you were able to travel. This value is then divided by the time you spent travelling this distance. The quality of the satellite signal will determine the accuracy of your sat nav that is not affected by your vehicle’s tyres.
Many sat navs may not be able to account for any changes when it comes to the vertical direction. Because of that, this may not be that accurate whenever you travel up or down any steep hill.
These sat navs can also inherently have better accuracy when they go at higher speeds because going for a farther distance over time can minimize the rounding errors. However, the sat nav can often be much closer to the car’s actual speed than the speedometer.
Some sat-nav systems improve their overall accuracy by using your car’s data to become integrated with the signal of the GPS.
GPS VS Speedometers
Since speedometers have a lot of mechanical parts, they can be significantly prone to mechanical errors. Speedometers are also measuring speed through indirect contact with the tyres.
So, when there are any changes in the property of the tires, it can lead to inaccuracy in reading. A typical example of this is the change in the tires’ dimension followed by load, temperature, tire pressure, and the usual wear and tear.
Manufacturers also calibrate the speedometers of the tyres that they install. However, whenever their customer works on changing the pre-installed tyres and uses different tyres from what they officially recommend, the speedometer would have to become recalibrated, or the speed reading will become inaccurate.
Since safety is of utmost priority, manufacturers have their speedometers calibrated to read higher than the vehicle’s actual speed. Whenever there are higher value alerts for speed, it will prevent the driver from speeding.
Yet, there is a limit to how low or high the speedometer will be allowed to read this. Speedometers are not permitted to read lower than the car’s actual speed, but they may indicate a 10% higher value than the actual speed with four added to it.
On the other hand, GPS uses a purely mathematical technique to measure the vehicle’s speed. Because of that, it can be more accurate compared to a speedometer. However, the GPS can also come with a catch.
These devices work best whenever the vehicle travels in a straight line with an uninterrupted sky view. It is vital to have a clear picture because the GPS receiver interacts with the satellites. Furthermore, a GPS device may not provide the vehicle’s instantaneous speed as it only provides the average speed with some slight delays.
The law for car speedometers in the UK
When it comes to UK Law, it is based on the EU standard that has some minor changes. The speedometer should never show less than the actual speed, and these must never show an excess of 110% of the actual speed or +6.25 miles per hour.
Even if your actual speed is at 40 miles per hour, your speedometer may legally read up to 50.25 miles per hour, but it would never be less than 40 miles per hour. Because of that, if your speedometer reads that your speed is at 50 miles per hour, it means that you are not going any faster than 50 miles per hour. There is an excellent possibility of travelling at just 40 miles per hour.
Make sure that you comply with the law by ensuring that your speedometers never show less than the vehicle’s actual speed under whatever foreseeable circumstance.
Typically, car manufacturers deliberately have their speedometers calibrated to read ‘high’ according to a specific amount. Since your sat nav is not a designated device that measures the speed of your car, any fudge factoring does not have to be incorporated.
Conclusion
To be honest, there is no considerable difference in the accuracy between a GPS device and a speedometer. For example, the speedometer may tell that your vehicle is going as fast as 63 miles per hour, but you are only going at 50 miles per hour.
The difference emerges because of the handicaps and mechanical errors that manufacturers have set. The speedometers that provide inaccurate readings and techniques are usually a blessing in disguise as these prohibit drivers from reckless driving, so they won’t get tickets all the time.
Also, make sure to check out our post on tips on how to use a sat nav when you go on a driving test so you can have a better chance of getting your driver’s license!
