Speed by speedometer and by GPS, difference

Article content

How Does a Car’s Speedometer Measure Speed?
How Mechanical and Electronic Speedometers Work
How Does GPS Measure a Car’s Speed?
Why Is Speed on the Speedometer Higher Than on GPS?
Which Is More Accurate – GPS or Speedometer?
How to Check a Speedometer’s Accuracy?
Conclusions

Many drivers have noticed that the speed displayed on a car’s speedometer often differs from the readings shown by a GPS device. This can be confusing and raises questions, especially for those using GPS for vehicle monitoring. Let’s explore the reasons behind this phenomenon and determine which speed measurement method is more accurate.

How Does a Car’s Speedometer Measure Speed?

The speedometer is a standard instrument that displays a car’s speed. It can be either mechanical or electronic, and their working principles differ.

How Mechanical and Electronic Speedometers Work

A mechanical speedometer operates using a flexible cable connected to the gearbox. As the car moves, wheel rotation transfers to the speedometer needle, displaying the current speed.

An electronic speedometer uses speed sensors installed in the transmission, which transmit data in digital format.

Factors Affecting Speedometer Accuracy

Even modern speedometers can have errors due to several factors:

Tire wear and size – A decrease in wheel diameter can lead to an overestimated speed reading.
Manufacturer calibration – Speedometers are usually set to show a slightly higher speed than actual.
Weather conditions – Low temperatures reduce tire pressure, altering their radius and affecting measurement accuracy.

Road incline – Speed readings may slightly fluctuate when driving uphill or downhill.

How Does GPS Measure a Car’s Speed?

GPS (Global Positioning System) calculates speed using data on the car’s movement between satellites.

How GPS Calculates Speed from Satellite Data

A GPS device records the vehicle’s position every few seconds and calculates speed based on the change in coordinates and time.

GPS Data Update Delay

Some devices may have a slight delay in updating information, especially in areas with weak signals (such as tunnels or among tall buildings). However, GPS generally provides highly accurate speed readings.

Factors Affecting GPS Accuracy

Number of satellites – The more satellites connected, the more accurate the data.
Signal quality – Bridges, tunnels, and buildings can interfere with signal reception.
Weather conditions – Heavy rain or snowfall can reduce measurement accuracy.

Why Is Speed on the Speedometer Higher Than on GPS?

The difference in readings is usually due to differences in speed measurement methods.

Manufacturers deliberately overestimate speedometer readings – International standards do not allow speedometers to show a speed lower than the actual value. Therefore, many manufacturers add 5–10% to the real speed.
Effect of wheel size – Worn or replaced tires can alter speedometer readings.
GPS errors – Possible in cases of weak signals, sudden maneuvers, or dense urban areas.

Technical malfunctions – Both speedometers and GPS systems may occasionally provide incorrect data.

Which Is More Accurate – GPS or Speedometer?

Both systems have their advantages and limitations:

GPS is usually more accurate, as it calculates speed based on actual movement.
Speedometers are more reliable in tunnels and areas with weak GPS signals.
The frequency of GPS data updates varies – professional tracking systems update more frequently than standard navigation devices.
The speedometer always functions, while GPS works only when the device is turned on.

How to Check a Speedometer’s Accuracy?

There are several ways to verify speedometer accuracy:

Comparison with a GPS device – You can use a navigator or a tracker for speed measurements.
Using mobile applications – Modern smartphones allow measuring speed using GPS.

Testing on marked roads – In some countries, highways have control markers that allow comparing actual speed with the speedometer readings.

Conclusions

Speed measurement methods have their specific features. The speedometer often overestimates speed, whereas GPS provides more accurate data but may have a delay in updates.

For the most accurate speed control, GPS monitoring is the best option, especially in logistics and fleet management. GPS systems allow not only speed tracking but also route recording, stop monitoring, and detecting possible violations.

Drivers should keep in mind that speedometers may not always be accurate, so when driving on roads with speed limits, it’s best to rely on multiple sources of information.