Why your speedometer reads high, and why GPS doesn't

Set a GPS speed app running on the passenger seat and watch it next to your dashboard for ten minutes. The two rarely agree. The dashboard is almost always the higher of the pair, usually by a few km/h, and the gap widens as you go faster.

Nothing is broken. Your car is behaving exactly as the law requires.

The regulation that makes it happen

Speedometers in most of the world outside North America are type-approved against UN Regulation No 39, the UNECE rule covering speedometer and odometer equipment. Its accuracy requirement, at paragraph 5.3, is short and one-sided:

The speed indicated shall not be less than the true speed of the vehicle.

And the permitted range is given as an inequality between the displayed speed V1 and the true speed V2:

0 ≤ (V1 – V2) ≤ 0,1 V2 + 4 km/h

Read that carefully, because the asymmetry is the whole story. The lower bound is zero. A car may never tell you that you are going slower than you actually are. The upper bound is generous: a tenth of your true speed, plus another 4 km/h on top.

What the allowance works out to

At a true 100 km/h, a dashboard showing 114 km/h is fully compliant. Most manufacturers do not use the whole allowance — a typical modern car sits a few percent high rather than the full fourteen — but they all deliberately sit somewhere above true, because the cost of drifting below zero error is failing type approval.

Why the rule is built this way

A speedometer is a liability instrument as much as an information one. If a car could under-read, a driver holding a steady indicated 70 could be doing 74 and receive a ticket for a fault in the vehicle. Regulators removed that possibility by outlawing the under-read entirely, and then allowed enough headroom above it that manufacturing tolerance, tyre wear and different approved tyre sizes can never push a car across the zero line.

That headroom has to absorb real physical variation. A car speedometer does not measure speed directly; it counts wheel or transmission rotation and multiplies by an assumed rolling circumference. That circumference changes with tyre pressure, with load, with temperature, and steadily as the tread wears down — a worn tyre is smaller, turns more times per kilometre, and pushes the indicated speed up further. Every one of those effects has to stay inside the band, so the band starts biased high.

Where GPS differs

A GPS speed reading is derived from your position and velocity relative to the satellite constellation. There is no wheel, no assumed circumference, no tyre wear, and no regulatory instruction to bias the number upward. What you get is an estimate of your actual rate of travel over the ground.

That is not the same as saying it is perfect. GPS speed degrades in the situations you would expect:

• Poor sky view. Tunnels, multi-storey car parks, urban canyons between tall buildings, and heavy tree cover all cut the number of satellites in view.
• Cold start. The first seconds after the app opens, before the fix settles, are the least reliable.
• Very low speeds. Near walking pace, position noise is large relative to the distance travelled, so the reading can wander.
• Sharp changes. Hard acceleration and braking are smoothed slightly by the update rate.

In steady cruising with a clear view of the sky — which is most driving — a GPS reading is generally closer to your true speed than the dashboard is.

What this means in practice

The practical consequence is a small margin most drivers never think about. If your dashboard reads 5% high and you sit at an indicated 70 mph on a motorway, your true speed is nearer 67. You are not gaining anything by it, and on a long journey you arrive slightly later than the dashboard implied.

It also means the dashboard is the wrong instrument for judging how close you are to a limit, in either direction. It will not let you exceed a limit while believing you are under it, which is the protection it was designed to give. But it will also quietly cost you a few km/h of the limit you are legally entitled to use.

Two things worth keeping straight:

Do not treat the gap as spare headroom. Enforcement measures your true speed, not your dashboard. If your car reads 5% high and you decide to sit 5% above the limit to compensate, you are at the limit exactly — with no margin left for a car that happens to read closer to true than you assumed.

Do not treat a phone as evidence. Consumer GPS is not a certified measuring instrument. It tells you something useful about your own driving. It is not a defence.

The odometer footnote

The same regulation covers odometers, and it is worth knowing that the distance figures inherit the same wheel-rotation assumptions. If your tyres are worn or non-standard, both your indicated speed and your recorded distance shift together. A GPS-derived trip distance is measured a different way and will not necessarily match the number on the dash.

This guide explains a regulation. It does not authorise exceeding any posted speed limit, and the limit that applies to you is the posted one, measured at your true speed.