Have you ever noticed that your sat nav speed is slightly different from what your car’s speedometer shows?
It’s a common question among drivers. In many cases, a sat nav will show a slightly lower speed than your car dashboard. This difference can make people wonder which one is actually correct.
The reason comes down to how each system measures speed. Your car and your sat nav use completely different technologies to calculate it.
Below, we explain why speed readings can differ and which one you should rely on while driving.
Why does my sat nav show a different speed?
Your car’s speedometer and your sat nav (GPS device) measure speed in different ways.
How a car speedometer works
A vehicle’s speedometer calculates speed by measuring how fast the wheels are rotating.
The system counts rotations from components such as the:
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Driveshaft
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Gearbox
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Wheels
It then calculates your speed based on the rolling circumference of your tyres.
Because this system relies on tyre size and rotation, the reading can change if:
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Your tyres wear down over time
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Different tyre sizes are fitted
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Tyre pressure changes slightly
Manufacturers also intentionally build in a small safety margin, meaning your speedometer usually reads slightly higher than your actual speed.
How sat nav devices calculate speed
Sat nav devices use GPS (Global Positioning System) to calculate speed.
Instead of measuring wheel rotation, the device:
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Tracks your position using satellites
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Measures the distance travelled between GPS points
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Calculates how quickly you moved between those points
Because GPS tracks real-world movement across the ground, it can sometimes produce a very accurate estimate of your true speed, particularly when driving at a constant speed on straight roads.
Which is more accurate: sat nav or speedometer?
In many situations, a sat nav speed reading may be closer to your true speed.
However, GPS speed readings can still be affected by factors such as:
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Weak GPS signal
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Tall buildings or tunnels
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Driving on winding roads
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Sudden acceleration or braking
Because of these variables, GPS devices are designed primarily for navigation, not precise speed measurement.
Which speed should you trust when driving?
Even if your sat nav appears more accurate, you should always rely on your car’s speedometer while driving.
This is because:
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Speedometers are designed to never show a speed lower than your real speed
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Speed limits and enforcement are based on vehicle speedometer readings
In other words, if your speedometer says 30 mph, your actual speed is likely slightly lower, helping ensure you stay within the legal limit.
Sat nav devices should, therefore, be treated as a useful guide but not your primary reference for speed.
Why do car speedometers overread?
UK and EU regulations require that speedometers must not display a speed lower than the vehicle’s actual speed.
To comply with these rules, manufacturers intentionally calibrate speedometers to slightly overestimate speed. This protects drivers from accidentally exceeding the speed limit due to small measurement errors.
If your sat nav speed and speedometer don’t match, there’s usually nothing wrong with your car.
The difference happens because:
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Speedometers measure wheel rotation
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Sat navs calculate speed using GPS location data
While GPS readings can sometimes be closer to your real speed, drivers should always rely on the vehicle’s speedometer to stay within speed limits.
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