How accurate is a GPS speedometer?

A phone GPS speedometer can provide a useful speed estimate in good reception, but no single mph or km/h error limit applies to every phone or trip. In NitroHUD, a label such as GPS ±5 m describes estimated position accuracy, not speed accuracy. To judge a reading, check whether it is fresh, whether reception is obstructed, and whether you are cruising steadily or changing speed.

What the accuracy number in meters means

Think of GPS ±5 m as an estimated circle around the reported position, with a radius of five meters. It does not mean the phone is exactly five meters from its true location, and it does not mean your speed is wrong by five mph, five km/h, or five meters per second. NitroHUD rounds the location provider’s accuracy value for display. The number describes horizontal position uncertainty; it is not an altitude error or a measurement against a known reference. A smaller value indicates a tighter estimated area, but it cannot prove that a particular reading is correct.

Why position accuracy and speed accuracy differ

A location provider can supply speed separately from position. Android’s documentation explains that this speed may use satellite Doppler measurements and can be more accurate than dividing the distance between two positions by elapsed time. NitroHUD uses the provider’s speed when available. When it is missing, the app estimates speed from successive positions instead, so position noise directly affects that fallback. NitroHUD does not display a separate speed uncertainty in mph or km/h. Its meter label cannot support a promise such as accurate to within 1 mph.

When a speed reading is more useful

Use fresh readings during steady travel with a clear view of the sky as a practical check. Watch for consistency across several updates rather than treating one number as a verdict. A small position radius, continuing updates, and a stable speed together give you more context than the radius alone. Even then, you have an estimate rather than a calibrated reference measurement. NitroHUD identifies itself as a driving information aid, not a certified speedometer. Keep using the vehicle’s instruments and set up the phone before moving.

Why buildings, trees, and tunnels cause trouble

GPS.gov identifies blocked signals, indoor or underground use, and reflections from buildings or walls as common causes of positioning errors. Reflections are called multipath: a signal reaches the receiver along an indirect route. Satellite geometry and receiver design also affect the result. A reading can therefore become less dependable downtown, under heavy cover, or in a tunnel. An accuracy value that suddenly grows is a reason to reassess reception. It does not identify the exact cause, and a value that stays small cannot rule out a bad position.

Why the display can lag or read zero at low speed

NitroHUD smooths the displayed speed, so it can trail the provider’s latest speed while you accelerate or slow down. It also treats speeds below 0.7 m/s, about 2.5 km/h or 1.6 mph, as zero. That is an app threshold, not proof that the vehicle is stationary. There is another limitation when the provider supplies no speed: small changes between positions are filtered out. Slow movement can then be missed even with a plausible accuracy label. For example, the current fallback can report zero when successive movements remain below that filter’s threshold.

What NitroHUD does with a large accuracy value

The current app does not reliably turn poor position accuracy into an explicit Weak signal message. It can continue showing a caption such as GPS ±90 m alongside a speed number. Internally it marks accuracy values above 60 m as weak, but that status alone does not blank the speed display. Distance counting uses a stricter position threshold of 25 m, together with movement checks. These thresholds use the unrounded value. They are app rules, not certified accuracy bands. A speed number remaining visible does not mean that the app has verified it, and a distance filter does not guarantee every bad position jump is rejected.

What Last fix means when updates stop

After more than six seconds without a new fix, NitroHUD can replace the accuracy caption with Last fix followed by the elapsed seconds. While that stale state is shown, the displayed speed is gradually reduced toward zero. That falling number is not a new measurement of the vehicle slowing down. If a location error occurs instead, the app may show GPS unavailable or Location is off and blank the gauge. In a tunnel, or whenever the last-fix age keeps rising, treat current GPS speed as unknown until fresh updates return.

A practical check before relying on the number

While parked, confirm that location access is enabled and that DEMO DRIVE is off; the demo uses simulated readings. Give the phone a clear view of the sky and allow real updates to arrive. Once traveling, have a passenger check the caption if needed. If the estimate worsens under cover, reassess after returning to open sky instead of changing settings while moving. If readings remain implausible there, check location permission and try a different secure phone position when stopped. NitroHUD requests high-accuracy location, but that request cannot guarantee the device will deliver it.