Error
Most speedometers have tolerances of some 10% plus or minus mainly due variation in tires diameter.[citation needed] Sources of error due to tire diameter variations are wear, temperature, pressure, vehicle load, and nominal tire size.
Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the
percent error = 100x(1 - "new diameter"/"standard diameter").
Nearly all tires now have their size shown as "T/A_W" on the side of the tire (See: Tire code), and the tire's
diameter in inches = TxA/1270 + W.
For example, a standard tire is "185/70R14" with diameter = 185x70/1270 + 14 = 24.20 in. Another is "195/50R15" with 195x50/1270 + 15 = 22.68 in. Replacing the first tire (and wheels) with the second (on 15" wheels), a speedometer reads 100 * (1 - 22.68/24.20) = 6.28% higher than the actual speed. At an actual speed of 60 mph, the speedometer will indicate 60 * 1.0628 = 63.77 mph, approximately.
In the case of wear, a new "185/70R14" tyre of 24.4 inch diameter will have ~8mm tread depth, at legal limit this reduces to 1.6mm, the difference being 12.8mm in diamter or 0.5 inches which is 2% in 24.4 inches.
International agreements
In many countries the legislated error in speedometer readings is ultimately governed by the United Nations Economic Commission for Europe (UNECE) Regulation 39[3] which covers those aspects of vehicle type approval which relate to speedometers. The main purpose of the UNECE regulations is to facilitate trade in motor vehicles by agreeing uniform type approval standards rather than requiring a vehicle model to undergo different approval processes in each country in which it is to be sold.
European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers [4] [5][6] are similar to the UNECE regulation in that they specify that:
* The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
* The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.
The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process, for example that the test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at a particular ambient temperature. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle.
The UNECE regulation relaxes the requirements for vehicles mass produced following type approval. At Conformity of Production Audits the upper limit on indicated speed is increased to 110 percent plus 6 km/h for cars, buses, trucks and similar vehicles, and 110 percent plus 8 km/h for two or three wheeled vehicles which have a maximum speed above 50 km/h (or a cylinder capacity, if powered by a heat engine, of more than 50 cc). European Union Directive 2000/7/EC, which relates to two and three wheeled vehicles, provides similar slightly relaxed limits in production.
Australia
There were no design rules in place for speedometers in Australia prior to July 1988. They had to be introduced when speed cameras were first used. This means there are no legally accurate speedometers for these older vehicles. All vehicles manufactured on or after 1 July 2007, and all models of vehicle introduced on or after 1 July 2006, must conform to UNECE Regulation 39.[7]
The speedometers in vehicles manufactured before these dates but after 1 July 1995 (or 1 January 1995 for forward control passenger vehicles and off-road passenger vehicles) must conform to the previous Australian design rule. This specifies that they need only display the speed to an accuracy of +/- 10% at speeds above 40 km/h, and there is no specified accuracy at all for speeds below 40 km/h. All vehicles manufactured in Australia or imported for supply to the Australian market must comply with the Australian Design Rules.
[8] The state and territory governments may set policies for the tolerance of speed over the posted speed limits that may be lower than the 10% in the earlier versions of the Australian Design Rules permitted, such as in Victoria.[9] This has caused some controversy since it would be possible for a driver to be unaware that he is speeding should his vehicle be fitted with an under-reading speedometer. [10]
United Kingdom
A speedometer showing mph and km/h along with an odometer and a separate 'trip' odometer (both showing distance traveled in miles).
The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39.[11]
The Motor Vehicles (Approval) Regulations 2001[12] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.
United States
As of 1997, Federal standards in the United States allowed a maximum 5% error on speedometer readings.[13] Aftermarket modifications, such as different tire and wheel sizes or different differential gearing, can cause speedometer inaccuracy.
GPS
GPS devices are capable of showing speed readings based on how far the receiver has moved since the last measurement. As the GPS is an independent* system, its speed calculations are not subject to the same sources of error as the vehicle's speedometer. Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change is lower. The GPS software may also use a moving average calculation to reduce error.
