Speed converter and speedometer difference calculator
Type a speed in any box and the others update. Below that, compare your dashboard with GPS to see how far it reads high.
Convert a speed
Dashboard versus GPS
Read both at a steady speed on a straight, level road. Have a passenger do it, not the driver.
- Difference
- Percent of GPS speed
- Highest reading R39 allows here
Everything runs in your browser. Nothing you type leaves this page.
The formulas
Each unit is a fixed number of metres per second, so a conversion goes through m/s:
| Unit | Exact definition | In m/s |
|---|---|---|
| km/h | 1000 m per hour | 0.27778 |
| mph | 1609.344 m per hour | 0.44704 |
| knot | 1852 m per hour | 0.51444 |
For the dashboard check, the difference is dashboard minus GPS, and the percent is that difference divided by the GPS speed.
UN Regulation No. 39 sets the allowed range. With V1 the shown speed and V2 the true speed, both in km/h:
0 ≤ V1 − V2 ≤ 0.1 × V2 + 4
The speedometer may never show less than true speed. It may show more, by up to 10% plus 4 km/h. The 4 is in km/h in every case, which is about 2.49 mph, and the calculator converts it for you.
Worked examples
In km/h. The dashboard shows 110 and GPS shows 105. The difference is 5 km/h, and 5 / 105 is 4.8%. The highest reading R39 allows at a true 105 km/h is 1.1 × 105 + 4 = 119.5 km/h, so 110 is inside the range. To estimate true speed near this speed, multiply the dashboard by 105 / 110, or 0.955.
In mph. The dashboard shows 70 and GPS shows 67. The difference is 3 mph, which is 4.5%. A true 67 mph is 107.8 km/h, and 1.1 × 107.8 + 4 = 122.6 km/h, or 76.2 mph. Using a plain 4 mph here would give 77.7 mph, which is wrong.
Limits
- GPS is not perfect. It is usually within about 1 km/h with a good fix, so the calculator treats smaller gaps as a match. When GPS speed is less reliable.
- One reading isn't the whole picture. The gap usually grows with speed, so the multiply-by number only holds near the speed you tested. Check a few speeds.
- It doesn't judge your car. R39 covers type approval of vehicles as built. Tyres change the reading later on, and this page can't tell you whether a vehicle complies with any law.
- Always follow the speed limit and watch the road. Don't read a phone while driving.
Common speeds
Rounded to one decimal.
| km/h | mph | knots | m/s |
|---|---|---|---|
| 10 km/h | 6.2 mph | 5.4 kn | 2.8 m/s |
| 20 km/h | 12.4 mph | 10.8 kn | 5.6 m/s |
| 30 km/h | 18.6 mph | 16.2 kn | 8.3 m/s |
| 50 km/h | 31.1 mph | 27.0 kn | 13.9 m/s |
| 60 km/h | 37.3 mph | 32.4 kn | 16.7 m/s |
| 80 km/h | 49.7 mph | 43.2 kn | 22.2 m/s |
| 100 km/h | 62.1 mph | 54.0 kn | 27.8 m/s |
| 110 km/h | 68.4 mph | 59.4 kn | 30.6 m/s |
| 120 km/h | 74.6 mph | 64.8 kn | 33.3 m/s |
| 130 km/h | 80.8 mph | 70.2 kn | 36.1 m/s |
| mph | km/h | knots | m/s |
|---|---|---|---|
| 20 mph | 32.2 km/h | 17.4 kn | 8.9 m/s |
| 30 mph | 48.3 km/h | 26.1 kn | 13.4 m/s |
| 40 mph | 64.4 km/h | 34.8 kn | 17.9 m/s |
| 50 mph | 80.5 km/h | 43.4 kn | 22.4 m/s |
| 60 mph | 96.6 km/h | 52.1 kn | 26.8 m/s |
| 70 mph | 112.7 km/h | 60.8 kn | 31.3 m/s |
| 80 mph | 128.7 km/h | 69.5 kn | 35.8 m/s |
Frequently asked
How do I convert km/h to mph? Divide by 1.609344, or multiply by about 0.621371. So 100 km/h is about 62.14 mph.
How fast is a knot? One knot is one nautical mile per hour, exactly 1.852 km/h, or about 1.151 mph.
Why does my speedometer read higher than GPS? Car speedometers are allowed to over-read and not to under-read. Under UN Regulation No. 39 the shown speed may be up to 10% plus 4 km/h above true speed. Makers usually set them a few percent high to stay on the safe side.
Which is right, GPS or the dashboard? With a good fix and a steady speed, GPS is usually within about 1 km/h of true speed. A dashboard counts wheel turns, so tyre size and wear change what it shows. In tunnels or at very low speeds, GPS is less reliable.
Does R39 apply to my car? It applies in Europe and many other countries that use UN vehicle regulations, for vehicles as approved when new. Other rules apply elsewhere, and a change of tyres can move the reading after that. The result here is a guide, not a legal or roadworthiness test.
Does this page send my numbers anywhere? No. The calculation runs in your browser and nothing you type leaves it.