Editing Speedometer (section)

== Error ==
Most speedometers have tolerances of some ±10%, mainly due to variations in tire diameter.{{Citation needed|date=June 2010}} Sources of error due to tire diameter variations are wear, temperature, pressure, vehicle load, and nominal tire size. Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits.{{Citation needed|date=August 2012}}

Excessive speedometer errors after manufacture can come from several causes, but most commonly is due to nonstandard tire diameter, in which case the error is:
<math display="block"> \mbox {Percentage error} = 100\times\left(1 - \frac \mbox{new diameter} \mbox{standard diameter}\right) </math>

Nearly all tires now have their size is shown as "T/A_W" on the side of the tire (See: [[Tire code]]), and the tires.

<math display="block"> \mbox {Diameter in millimetres} = 2 \times T \times A / 100 + W \times 25.4 </math>
<math display="block"> \mbox {Diameter in inches} = T \times A / 1270 + W </math>

For example, a standard tire is "185/70R14" with diameter = 2*185*(70/100)+(14*25.4) = 614.6&nbsp;mm (185x70/1270 + 14 = 24.20 in). Another is "195/50R15" with 2*195*(50/100)+(15*25.4) = 576.0&nbsp;mm (195x50/1270 + 15 = 22.68 in). Replacing the first tire (and wheels) with the second (on 15" = 381&nbsp;mm wheels), a speedometer reads 100 * ((614.6/576) - 1) = 100 * (24.20/22.68 - 1) = 6.7% higher than the actual speed. At an actual speed of 100&nbsp;km/h (60&nbsp;mph), the speedometer will indicate 100 x 1.067 = 106.7&nbsp;km/h (60 * 1.067 = 64.02&nbsp;mph), approximately.

In the case of wear, a new "185/70R14" tire of 620&nbsp;mm (24.4&nbsp;inch) diameter will have ≈8&nbsp;mm tread depth, at legal limit this reduces to 1.6&nbsp;mm, the difference being 12.8&nbsp;mm in diameter or 0.5&nbsp;inches which is 2% in 620&nbsp;mm (24.4&nbsp;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,<ref>{{cite web|url= http://www.unece.org/trans/main/wp29/wp29regs21-40.html |title=UNECE Transport Division – Vehicle Regulations – Addenda to 1958 agreement – Regulations 21–40 |publisher=UN Economic Commission for Europe |access-date=30 January 2015}}</ref> which covers those aspects of vehicle type approval that relate to speedometers. The main purpose of the UNECE regulations is to facilitate trade in motor vehicles by agreeing on uniform type approval standards rather than requiring a vehicle model to undergo different approval processes in each country where it is sold.

[[European Union]] member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers<ref>{{cite web|url= http://eur-lex.europa.eu/eli/reg/2004/39(2)/oj |title=Regulation No 39 of the Economic Commission for Europe of the United Nations (UN/ECE) — Uniform provisions concerning the approval of vehicles with regard to the speedometer equipment including its installation |work=European Commission |access-date=8 April 2017}}</ref><ref>{{cite web |url= http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31997L0039:EN:NOT | title = Commission Directive 97/39/EC of 24 June 1997 adapting to technical progress Council Directive 75/443/EEC of 26 June 1975 relating to the reverse and speedometer equipment of motor vehicles |work=European Commission | date = 24 June 1997 |access-date=7 January 2007}}</ref><ref>{{cite web |url= http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32000L0007:EN:NOT | title=Directive 2000/7/EC – speedometers for two- or three-wheel motor vehicles |work=European Commission | date=20 March 2000 |access-date=7 January 2007}}</ref> 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 {{cvt|4|km/h}} at specified test speeds. For example, at {{cvt|80|km/h}}, the indicated speed must be no more than {{cvt|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, the test measurements should be made (for most vehicles) at {{cvt|40|,|80|and|120|km/h}}, and at a particular ambient temperature and road surface. 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 {{cvt|6|km/h}} for cars, buses, trucks, and similar vehicles, and 110 percent plus {{cvt|8|km/h}} for two- or three-wheeled vehicles that have a maximum speed above {{cvt|50|km/h}} (or a cylinder capacity, if powered by a [[heat engine]], of more than {{cvt|50|cm3}}). 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 [[Australian Design Rules]] in place for speedometers in Australia before 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.<ref>{{cite web | title = Australian Design Rule 18/03 – Instrumentation |work=Road Vehicle Certification System |url= http://rvcs-prodweb.dot.gov.au/files/ADR%201803.pdf |access-date=7 January 2008 }}</ref>

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&nbsp;km/h, and there is no specified accuracy at all for speeds below 40&nbsp;km/h. 

All vehicles manufactured in Australia or imported for supply to the Australian market must comply with the Australian Design Rules.<ref>{{cite web |url= http://www.comlaw.gov.au/ComLaw/Legislation/LegislativeInstrument1.nsf/0/A678C717ABCB8D02CA2571CC0014D003?OpenDocument | title=Australian Design Rule 18/02 – Instrumentation | work = Commonwealth of Australia Law | access-date=14 January 2008}}</ref> 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.<ref>{{cite web|url = http://www.aussiemotorists.com/misc/msa-speedo.html | title = Vehicle Speed Measurement II |work=National Motorists Association Australia |year=2004 |first=Leslie |last=Felix |access-date=14 January 2008}}</ref> This has caused some controversy since it would be possible for a driver to be unaware that they are speeding should their vehicle be fitted with an under-reading speedometer.<ref>{{ cite web|url = http://www.parliament.vic.gov.au/rsc/DEMERIT/demerit3.htm#6 |work=Victoria Road Safety Committee, Inquiry into the Demerit Points Scheme |date=November 1994 |title=3.6 Accuracy of speedometers |access-date=14 January 2008}}</ref>

===United Kingdom===
[[File:Ford Mondeo MK3 ST220 - Speedometer (light).jpg|thumb|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.<ref>{{cite web |
url= https://publications.parliament.uk/pa/ld200001/ldhansrd/vo010312/text/10312w01.htm |title=Speedometer Accuracy | work=Written Answers, Hansard (UK Parliament proceedings) Monday, 12th March 2001 |access-date=7 January 2008}}</ref>

The Motor Vehicles (Approval) Regulations 2001<ref>{{cite web |url= http://www.opsi.gov.uk/si/si2001/20010025.htm#sch3 |title=The Motor Vehicles (Approval) Regulations 2001: Schedule 3 |work=Office of Public Sector Information |access-date=19 December 2007}}</ref> 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&nbsp;mph and 70&nbsp;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&nbsp;mph.

For example, if the vehicle is actually traveling at 50&nbsp;mph, the speedometer must not show more than 61.25&nbsp;mph or less than 50&nbsp;mph.

===United States===
Federal standards in the [[United States]] allow a maximum 5&nbsp;mph error at a speed of 50&nbsp;mph on speedometer readings for commercial vehicles.<ref>{{cite web|url= https://www.ecfr.gov/cgi-bin/retrieveECFR?gp=1&ty=HTML&h=L&mc=true&=PART&n=pt49.5.393#se49.5.393_182 |title=eCFR – Code of Federal Regulations |website=ecfr.gov |access-date=18 February 2019}}</ref> Aftermarket modifications, such as different tire and wheel sizes or different differential gearing, can cause speedometer inaccuracy.