Why Your Speedometer Reads Higher Than Your Real Speed
Set the cruise control to 100 and a satellite-based reading often shows something closer to 95. Your car is not faulty. It is behaving exactly as the rules require.
Understanding this gap explains a lot about how drivers behave in average-speed zones — including why so many of them are slower than they believe.
The rule behind it
European type-approval rules require that a speedometer must never display less than the true speed of the vehicle. It is allowed to display more, within a defined margin that scales with speed.
In other words, the error is deliberately one-directional. A car that under-reads would be illegal; a car that over-reads is normal, and virtually every manufacturer builds in some margin.
Why manufacturers keep a margin
Wheel and tyre dimensions change the picture. A worn tyre has a slightly smaller rolling radius than a new one, and a legal replacement size can differ slightly from the original. Both shift the relationship between wheel rotation and real speed.
Rather than risk ever showing a number below reality, manufacturers set the display a little high and leave it there for the life of the car.
What it means inside an average-speed zone
If your dashboard shows a number above your true speed, then holding the dial exactly on the limit means your real average ends up slightly below it. That is safe from an enforcement point of view — and it is also why long queues form behind cautious drivers in these sections.
The temptation is to reason backwards and treat the gap as spare room. Do not. The limit applies to your true speed, the margin is not guaranteed on any particular car, and a measured average does not care what your dashboard displayed.
Satellite speed versus the dial
A satellite-derived speed is calculated from position over time, so it reflects ground speed rather than wheel rotation and is usually closer to the truth. It can wobble in tunnels, deep valleys or dense city canyons, where the signal degrades.
This is the practical value of an app that computes your average from your actual movement: the section is judged on distance and time, so distance and time are exactly what you want to see while driving it.
Frequently asked questions
Is my speedometer broken if it reads a few percent high?
Almost certainly not. Over-reading is permitted within a defined margin, while under-reading is not permitted at all, so a small positive offset is the expected behaviour.
Which should I trust, the dial or a satellite reading?
A satellite-based reading is normally closer to your true ground speed, but it can degrade where the signal is weak. Treat the dial as the conservative number and never assume the gap is available to use.
Does this mean I can safely exceed the posted limit?
No. Limits apply to true speed, the margin varies between vehicles and tyre conditions, and enforcement measures your real average. The gap is a safety buffer, not an allowance.