Written by Muneeb Rana · Last updated 26 October 2026
Quick answer: Why car speedometers read high: mainly because the rules forbid them from ever showing less than true speed. Under UN ECE Regulation 39 the display may over-read by up to 10% plus 4 km/h, so makers aim a little high to stay safe across tyre sizes, wear and manufacturing tolerances. The result is that you are usually going.
TL;DR
- Regulations forbid under-reading.
- Allowed over-read: up to 10% + 4 km/h under R39.
- Tyre wear and pressure add variation.
- Makers build in a safety margin.
- GPS gives a useful cross-check.
Why car speedometers read high: the regulation explained
UN ECE Regulation 39 sets the rule many countries use: 0 ≤ indicated − true ≤ 0.1 × true + 4 km/h. At a true 50 km/h the car may show up to 59 km/h; at a true 100 km/h, up to 114 km/h. A speedometer showing less than true speed fails.
Why makers aim high
A car's speedometer infers speed from how fast the wheels or gearbox turn. The real distance per turn depends on tyre diameter, which varies by brand, wear and inflation. Because under-reading is illegal, makers calibrate so even the largest permitted tyre will not under-read, which leaves most cars reading somewhat high.
Tyres move the needle
New tyres are a few millimetres taller than worn ones, so a car reads slightly higher as tyres wear. Fitting larger wheels without recalibrating can wipe out the margin or cause under-reading. Use our speedometer error calculator before changing sizes.
How to measure your own error
On a straight, open road with good sky view, hold a steady indicated speed and compare with a GPS speedometer such as Speedometer PRO. Have a passenger read it, or glance at a mounted phone; do not handle it. Repeat at 30, 50 and 70 mph. Full method in check speedometer accuracy.
Odometers over-read too
The odometer usually shares the same signal, so it may log slightly more distance than travelled. That matters for leases and mileage claims; see odometer vs trip meter.
Should you correct for it?
No. Drive by the speedometer; its over-read gives a small safety margin.
Is it safe to drive to GPS speed?
Use GPS as information, not permission. Posted limits, road conditions and enforcement devices are what count, and enforcement tolerances vary by place. When unsure, follow the dashboard.
Related: GPS speedometer guide · GPS vs car speedometer: which one is right? · Speedometer tolerance rules explained · Speedometer error calculator for tyre size changes · How to check speedometer accuracy · Tire size speedometer error explained
Get Speedometer PRO on Google Play · Download on the App Store · Set it up before you move; never handle your phone while driving.
Sources
- UN Regulation No. 39: speedometer and odometer (UNECE): unece.org (accessed 26 October 2026)
- Speedometer (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- Tire code (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- Odometer (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
FAQ
How much do car speedometers over-read?
Commonly a few percent. The legal ceiling under Regulation 39 is 10% of true speed plus 4 km/h, but most cars sit well inside that.
Can a speedometer legally read low?
Not under UN ECE Regulation 39, which requires indicated speed never to be below true speed.
Do new tyres change the speedometer?
Slightly. New tyres are larger in diameter than worn ones, so the speedometer reads a little lower than it did on worn tyres.
Does my odometer read high too?
Often slightly, because it uses the same wheel-speed signal and calibration as the speedometer.
Should I drive faster to match GPS?
No. Stick to the posted limit using your dashboard. GPS readings are estimates and can be wrong.