Written by Muneeb Rana · Last updated 26 October 2026
Quick answer: Speedometer tolerance rules set how far a car's displayed speed may differ from true speed. Under UN ECE Regulation 39 the indicated speed must never be below true speed and may exceed it by at most 10% plus 4 km/h. These are vehicle-approval rules for the instrument, not a speeding allowance; police enforcement tolerances are a separate matter.
TL;DR
- R39: never under-read.
- Max over-read = 10% of true + 4 km/h.
- Applies to type approval of vehicles.
- Not the same as police enforcement thresholds.
- US rules differ.
The speedometer tolerance formula
Regulation 39 is a UN vehicle regulation adopted by many countries. It states that indicated speed V1 and true speed V2 must satisfy 0 ≤ V1 − V2 ≤ 0.1 V2 + 4 km/h.
Worked examples
| True speed | Allowed display range |
|---|---|
| 30 km/h | 30–37 km/h |
| 50 km/h | 50–59 km/h |
| 80 km/h | 80–92 km/h |
| 100 km/h | 100–114 km/h |
| 120 km/h | 120–136 km/h |
United States
The US does not adopt R39. Federal Motor Vehicle Safety Standard 101 governs how controls and displays look and where they sit. Manufacturers still calibrate to avoid under-reading for liability reasons.
Enforcement is different
Police forces and camera systems apply their own thresholds before issuing tickets, and these vary by country and agency. Never assume a tolerance exists. The limit is the limit. See US speed limits for context and dangers of speeding.
Where GPS fits
A GPS speedometer lets you see your car's real-world margin. Read GPS vs car speedometer. It is an estimate too, not a certified instrument.
US rules
The US has no federal speedometer accuracy standard for passenger cars comparable to UN R39; manufacturers set tolerances.
What this means for you
A speedometer that reads a little high is normal. One that reads low is a fault worth having checked.
Aftermarket changes
Changing tyre or wheel size, or fitting a different gearbox ratio, can push a car outside its approved tolerance. Check with the speedometer error calculator and a qualified fitter.
Related: GPS speedometer guide · Why car speedometers read high · GPS vs car speedometer: which one is right? · Speedometer error calculator for tyre size changes · Odometer fraud: how to spot it · Speedometer app safety: how to use it without distraction
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)
- 49 CFR 571.101: controls and displays (eCFR): ecfr.gov (accessed 26 October 2026)
- Odometer Fraud (NHTSA): nhtsa.gov (accessed 26 October 2026)
- Speedometer (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
FAQ
What is the legal speedometer tolerance?
Under UN ECE Regulation 39, displayed speed must be between true speed and true speed plus 10% plus 4 km/h.
Does the tolerance mean I can drive 10% over?
No. It applies to how the instrument is built, not to how fast you may drive. Speed limits apply to your true speed.
Does R39 apply in the US?
No. The US uses its own federal standards, which do not adopt the R39 tolerance formula.
Can my speedometer be out of tolerance?
Yes, after tyre size changes, damage or faults. A GPS cross-check can reveal a large error.
Is there a tolerance for odometers?
Odometer rules differ from speedometer rules and vary by country. US law focuses on preventing tampering rather than setting an accuracy band.