The short answer: car speedometers must not under-read
Vehicle speedometers are required never to show a speed lower than the true speed, with a limit on how much higher they may read. So in many cars the speedometer reads a little above your true speed, which GPS measures more directly.
UN Regulation No. 39
UN Regulation No. 39 on speedometer equipment, from the UN Economic Commission for Europe (UNECE), says in paragraph 5.4:
- the speed indicated shall not be less than the true speed of the vehicle
- at the test speeds, the displayed speed V1 and the true speed V2 must satisfy 0 ≤ V1 − V2 ≤ 0.1 V2 + 4 km/h
So at a true 100 km/h the display may read anywhere from 100 to 114 km/h, and at a true 50 km/h, from 50 to 59 km/h. The test speeds depend on the vehicle's maximum speed; for vehicles faster than 150 km/h they are 40, 80 and 120 km/h.
Japan's vehicle safety standards
In Japan, Article 46 of the Safety Regulations for Road Vehicles requires a speedometer without significant error when driving on flat paved roads, and leaves the details to a notice from the Ministry of Land, Infrastructure, Transport and Tourism (MLIT).
- Type approval: Attachment 88 of the notice, "Technical standard for speedometers", is harmonized with UN Regulation No. 39. It says the reading must not be lower than the vehicle's actual speed and requires 0 ≤ V1 − V2 ≤ V2/10 + 4 (km/h). Outside type designation, the upper limit becomes V2/10 + 6 (+ 8 for motorcycles and some other vehicles) under Article 70 of the notice.
- Inspection: For vehicles built on or after 1 January 2007 (excluding motorcycles, three-wheelers and a few others), when the speedometer shows 40 km/h, the speed measured on a speedometer tester must satisfy 10 × (V1 − 6) / 11 ≤ V2 ≤ (100 / 94) × V1 (Articles 148 and 226 of the notice). That works out to a true speed of about 30.9 to 42.5 km/h for a reading of 40 km/h.
The idea is the same in both: reading a bit high is allowed within limits; reading low is not.
Other reasons for the gap
On the car side
The tests are run on the tires the manufacturer specifies, at a set pressure. In general, wear, pressure and tire size change the tire's outer diameter, which slightly changes how the reading relates to your true speed.
On the GPS side
GPS speed has its own error and lag. According to GPS.gov, smartphone GPS is typically accurate to within 4.9 m (16 ft) under open sky, but worse near buildings, bridges and trees, indoors, and with reflected signals (multipath). The reading lags slightly when you speed up or slow down, and it stops in tunnels. More in how accurate is GPS speed.
While driving, follow your car's speedometer
The driver watches the road and the car's own speedometer. Follow the traffic laws of the country you are in. In Japan, for example, Article 71, item 5-5 of the Road Traffic Act says that while driving a car, a moped or a bicycle, you must not gaze at images on a display device mounted on or brought into the vehicle, except when stopped, and violations carry penalties.
- GPS speed is a reference for passengers, in the front or back seat
- If the driver wants to look, stop somewhere safe first
- A gap of a few km/h between the car and GPS is usually within the normal range
Where there is no car speedometer at all, on a boat, a train or a run, GPS speed is a handy guide. For unit conversions, see our km/h, mph, knots and m/s conversion chart.
Sources
- UNECE: Addenda to the 1958 Agreement (Regulations 21-40), Regulation No. 39
- MLIT: Safety Regulations for Road Vehicles, Article 46 (Japanese)
- MLIT: Notice on detailed safety standards, Article 70 (Japanese)
- MLIT: Notice, Attachment 88, Technical standard for speedometers (Japanese)
- MLIT: Notice, Article 148 (Japanese)
- MLIT: Notice, Article 226 (Japanese)
- GPS.gov: GPS Accuracy
- e-Gov Laws (Japan): Road Traffic Act, Article 71, item 5-5 (Japanese)