Written by Muneeb Rana · Last updated 26 October 2026
Quick answer: To check speedometer accuracy, compare your dashboard with a GPS speedometer on a straight, open road at a steady speed, with a passenger reading the phone. Repeat at several speeds. A dashboard reading a few percent high is normal; one that reads low, or more than about 10% high, should be checked by a mechanic.
TL;DR
- Use a passenger; never handle the phone.
- Pick a straight road with open sky.
- Hold steady speed for 10+ seconds.
- Test 30, 50 and 70 mph.
- Low readings need attention.
What you need
A phone with Speedometer PRO or our online speedometer, a secure mount or a passenger, and a quiet straight road with clear sky. Let GPS settle for a minute before starting.
Check speedometer accuracy step by step
- Set up the app and mount before you set off.
- On the test road, use cruise control or a steady throttle at an indicated 30 mph.
- After 10 seconds, the passenger notes the GPS figure.
- Repeat at 50 and 70 mph (where legal).
- Calculate error: (dashboard − GPS) ÷ GPS × 100.
Mile-marker method
Without GPS, time a measured mile at a steady indicated 60 mph. At a true 60 mph one mile takes exactly 60 seconds. If it takes 62 s, true speed is 3600 ÷ 62 ≈ 58 mph. The average speed calculator does the arithmetic.
Reading the results
| Result | Meaning |
|---|---|
| Dashboard 0–5% high | Normal |
| Dashboard >10% high | Check tyres and calibration |
| Dashboard lower than GPS | Possible fault or oversize tyres; get it checked |
| Inconsistent gaps | Poor GPS conditions; retest |
Common causes of error
Non-standard tyre sizes, worn or under-inflated tyres, a replaced instrument cluster or a changed final-drive ratio. See tire size speedometer error.
Recording your results
Note the dashboard reading, the GPS reading and the conditions for three or four steady speeds, such as 30, 50 and 70 mph (or 50, 80 and 110 km/h). Repeat on another day. A consistent percentage difference points to the car's built-in over-read or tyre size; random differences suggest GPS conditions were poor. Do this with a passenger reading the phone, never yourself while driving.
Safety first
Run the test only where legal and safe, follow limits, and keep your eyes on the road. If alone, record nothing during the drive; let the mounted display speak for itself and stop somewhere safe to note results.
Related: GPS speedometer guide · GPS vs car speedometer: which one is right? · Why car speedometers read high · Average speed calculator · Speedometer error calculator for tyre size changes · Speedometer for car: your phone as a second display
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)
- GPS Accuracy (GPS.gov): gps.gov (accessed 26 October 2026)
- Speedometer (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- Tire code (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
FAQ
How accurate should my speedometer be?
Most read a few percent high. UN ECE rules allow up to 10% plus 4 km/h over true speed but never under.
Can I check speedometer accuracy alone?
Only with the phone mounted and set up beforehand, glancing as you would at a mirror. A passenger is much safer.
Is the mile-marker method accurate?
Reasonably, if markers are precise and speed is steady. Small timing errors matter, so average several runs.
What if GPS shows a higher speed than my dashboard?
Your speedometer may be under-reading, which is unsafe and in many countries non-compliant. Have it checked.
Does the test check my odometer too?
Indirectly. The same percentage error usually applies to logged distance, which you can confirm against a known route.