How are speedometers designed to accommodate the change from mph to km/h? In this article I will examine a variety of techniques and at the end of the article I will propose a 21st Century solution.
Australia and South Africa
I lived in South Africa when that country adopted the metric system. I believe that there were many similarities between the Australian and the South African approaches, so I will lump them together as a single approach. From what I can recall, cars sold up to and including 1971 had mph speedometers and those sold from 1972 onwards had km/h speedometers. There were no dual unit speedometers. Drivers were expected to “know” the conversion factors – the only one of real consequence was that the 35 mph urban speed limit became a 60 km/h speed limit. On the open road, the old 70 mph speed limit became 110 km/h, but was rarely enforced.
Various gadgets were available to help people understand the new system, in particular I remember an adaptor that one could insert between the speedometer cable and the speedometer itself which had the effect of speeding up the internal rotating cable by a factor of 1.6, making the mph speedometer into a km/h speedometer (a pity that it still read “mph” on the speedometer face).
Although the South African approach appeared a little rough at the edges, South African drivers were implicitly given the message “The country is going metric – just get used to the idea and stop clinging onto the imperial system”.
The Yom Kippur War of 1973 brought a big change when oil world-wide production dropped sharply. As a fuel conservation measure, the South African authorities introduced a regime of high fines for anybody breaking the reduced speed limits (50 km/h in urban areas and 80 km/h on the open road). Drivers with mph speedometers very quickly learnt how to convert into km/h!
United Kingdom and United States
The United Kingdom and the United States (and certain other countries) took the approach that drivers should not have to do mental conversions and so introduced dual unit speedometers with mph as the principal unit and km/h as a secondary unit. This change was introduced many years before the planned conversion dates, putting the onus on the car manufacturers to prepare for a conversion date at some unspecified time in the future.
On the surface this appears to be a solid compromise. Once the conversion took place, drivers would not have to remember the conversion factors and as the old cars were replaced, new cars would have km/h only speedometers. In theory this looked fine, but in practice it gave governments “wriggle room” – they could (and did) postpone metrication of road signs because the circumstances were not “quite right” (i.e. a “sterling crisis”, or an upcoming election). The result is that the conversion has not taken place and certain elements of society who have no real interest in units of measure hijacked the anti-metrication movement for their own agenda.
One of the design flaws (in my opinion) of some speedometers such as the one in the above diagram is that the lettering “MPH” and “km/h” are not adjacent to each other. I heard of an Australian driver who, seeing the “km/h”, but not noticing the “MPH” thought that the outer display was in km/h.
European Train Control System (ETCS)
In the 1990s, the EU expressed concern each country had its own railway signalling system (or in some cases multiple signalling systems) forcing high-speed international trains to carry a multitude of signalling devices for in-cab signals. Thus, the first Eurostars had to carry British, French and Belgian-compatible signalling systems on their high-speed (above 200 km/h) trains. As a result, the EU instructed the rail industry to develop a pan-European signalling system for high-speed trains where the drivers relied on in-cab displays rather than line-side displays. The result was ETCS which is now in use in many parts of Europe as well as certain routes in Australia, India, Philippines and many other countries.
Being a pan-European project, it is hardly surprising that it is all-metric.
ETCS relies on picking up information from standard units (known as “Eurobalises”) that are mounted on the railway sleepers. ETCS is designed to be introduced over a period of time and has different “levels” depending on the degree of sophistication that is incorporated into the local system. All levels use the same driver-machine interface (DMI) unit which consists of a large display unit and associated electronics (see above diagram).
- ETCS Level 0 is used where the train has a DMI installed to replace a legacy speedometer, but there is no associated lineside equipment and, as a result, the speedometer is the only ETCS component that is operational.
- ETCS Level 1 is used where lineside signals and in-cab information are used in tandem. As far as I can see, Level 1 will not be used in the United Kingdom.
- ETCS Level 2 is used when lineside signals are dispensed with and all the information that the driver needs is in the cab.
At the time of writing, ETCS Level 2 is operational on the Moorgate spur of the East Coast Main Line (ECML) and is being installed as far as Peterborough. Once that has been completed, trains will run under Level 2 from Kings Cross to Peterborough and under Level 0 onwards to Edinburgh.
In the United Kingdom, ETCS is programmed to display mph when operating under Level 0 and km/h when operating under Level 2 with the switchover occurring automatically. As can be seen from the above diagram, there is little confusion as whether the unit is displaying mph or km/h – there is no supplementary information at Level 0! One of the design features of the ETCS DMI is that the angular position of the needle is independent of the units being displayed – thus when the train goes from a Level 0 section of track to a Level 2 section of track, the needle itself will not change position.
A 21st Century Proposal
Both the South African/Australian car speedometers and the ETCS speedometer have the advantage of only displaying speeds in one system of units, thereby reducing clutter. In the case of the South African and Australian cars, drivers were required to mentally convert km/h back to mph if they were driving older cars, whereas ETCS switched between units as appropriate.
Many modern cars provide the driver with the facility to switch between mph and km/h on a digital display, but I have not seen any proposals to enable such switching to be done on an analogue (or pseudo-analogue) display. Drawing on features of the above, I am making the following proposal.
In the 1970s, speedometer backing plates were pieces of metal with numbers painted on them. The technology of the 21st century makes frequent use of liquid crystal displays (LCDs). They appear on every laptop computer, mobile phone and on a host of other devices. I suggest that the backing plates for switchable speedometers should use LCD technology, and that the driver would be able to select the units displayed at the flick of a switch. First and foremost, such a display would have to comply with UK (and therefore EU) rules, which in turn comply with the UNECE recommendations for speedometers. The recommendations for speedometers that do not go above 200 km/h include:
- It is mandatory that the speedometer shall be able to show km/h. The display of mph is optional (required for new cars sold in the United Kingdom). It is permitted to have switchable displays.
- Graduations shall be in multiples of 1, 2, 5 or 10 km/h [or mph].
- Speed values shall be in multiples not exceeding 20 km/h [or 20 mph].
- Graduations need not be evenly spaced.
- The indicated speed may not be less than the true speed.
As can be seen in the above diagram, the proposed speedometer complies with all these recommendations. The most important feature (borrowed from the ETCS specifications) is that the position of the needle is independent of speed. Thus, the speedometer could be a physical needle whose angle is directly proportional to the rotational speed of the wheels.
In the diagram, the text “mph” and “km/h” are in different colours. This could of course be extended to all characters on the display and even to the background colour. Such changes could be subtle, but sufficient to alert the motorist as to which units are being displayed. (Any students of ergonomics out there?).
The other feature that I have added to the display are the red lines. The first of these is at 25 mph – 40 km/h and will be in exactly the same position when switching between units. Thus, the driver will get used to seeing the speedometer needle a little above the first red line when driving in Dover at 30 mph or in Calais at 50 km/h. Subsequent red lines are at multiples of 25 mph and 40 km/h. Thus, drivers who take their cars across the Channel or who drive between Northern Ireland and the Republic could mentally tune themselves to using the red lines as a first approximation as to a safe speed.
One could of course use multiples of 30 mph – 50 km/h. This would have the advantage of the first red line showing the default urban speed unit in both Dover and Calais but would result in a thicker line (the ratio between 25 mph and 40 km/h being 1.006 and the ratio between 30 mph and 50 km/h being 1.034).
This proposal has the advantage that it would cost very little and would be of immediate use to British drivers when using their cars abroad.
Metric Views 
Minor correction on the US and Canada. Neither requires a dual unit speedometer. The US allows MPH-only or dual ring with MPH the primary (outer ring, larger font). Canada allows km/h-only or dual ring, with km/h as the outer ring. If dual ring, the scales must be labeled as MPH and km/h with the labels “clearly” associated with the correct scale by placement and/or color. Everyone uses different colors, usually white for the primary, some kind of orange or light gray for the minor. These requirements are detailed in FMVSS101, or the Canadian equivalent, CMVSS101.
The US has no metric speed limits and Canada has no non-metric (there may be some dual signage). Dual ring is primarily for the convenience of drivers who cross the border, but most vehicles have dual ring speedos.
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On a slightly related note (Imperial vs metric units displayed to a user or customer) it seems to me that the trade war being started by the USA with its immediate neighbours (Canada and Mexico) could push Canada more in the direction of metric if the tariffs persist.
Many American products are being removed from store shelves in Canada. Efforts are also underway to streamline trade amongst the provinces (which it has surfaced have unexpected trade barriers, surprisingly). If many products from the USA are either improperly labeled (showing US Customary only or more prominently than metric or the metric shown is just a weird-looking soft conversion from a rational US Customary quantity), then the lack of such non-metric only products or sloppy metric products in stores and improved inter-province trade should create a more metric only shopping experience for consumers, thus driving home a better daily familiarity with metric with a subsequent unwillingness to give up on a metric only shopping experience even after the tariffs are gone.
Moreover, many Canadians are very angry at the USA right now. In some quarters the conversion to metric there (though done many years ago) is a source of pride as a way of distinguishing themselves from the USA. The more Trump talks about making Canada the 51s state, the more likely Canadians will want to enhance their use of metric as a way of making themselves not American (especially in English Canada since Quebec already has the sharp line of demarcation of using French and getting so much of their information and culture from France and other French language sources and I think is already more aligned psychologically with metric units).
Many folks in Canada are also talking about setting up closer trade relationships with other countries, especially the EU. Also, if the UK finds that they cannot get accommodation from the USA, that could also add impetus for the UK to rejoin the EU (or at least the Customs Unions or even also the Single Market) and could possibly induce the government to think seriously about finishing metrication in the UK to complete the conversion, including road signs, so as to be better aligned with the EU.
Maybe I am grasping at straws, but I am good at that, I suspect. 😉
Ezra (aka punditgi)
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Ezra,
I’m hoping that the trade war becomes a huge blessing in disguise. It can work in the direction of greater use of SI units in Canada if as you say Canadians are becoming super mad at the US for the tariffs on Canadian goods sold in the US.
But, a trade war can only be a blessing towards SI completion in Canada if those Canadians who are die-hard supporters of SI completion take the opportunity to nag both business and government to take an active role in completing metrication, even to the point of pushing and encouraging average citizens to use only SI units in their daily lives for everything, by measuring and communicating all measurement information in SI units only.
The government can do a good deed by educating Canadians via the media to learn, get familiar with and use only SI units in their daily lives as a means to prove to the world that Canada is SI ready and that Canadians can communicate to the world in SI units and Canadian goods and services are SI based such that Canada ends it dependence on the US market. Canada may also have to go beyond just completing metrication, it must also reject all US based industrial standards and adopt all of the ISO and IEC technical standards for normal use in Canada. Failure to do so would make Canadian industrial products unsellable in much of the world.
The question is, what will it take for Canadians to wake up and smell the roses and be willing to make whatever sacrifices are needed to break Canada free from US control? I think a first step Canada must take is for both it and México to end the NAFTA trade agreement and apply to BRICS.
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An additional comment I need to make to my previous post is that since Donald Trump seems to think tariffs are a money maker for the US, Canada and other countries can use tariffs to enforce metrication. Any non-SI product sold in Canada would be taxed at a heavy level and any company still producing non-SI parts no matter what the reason would have those products taxed as well. The taxes collected could be put into a fund to be used to pay for educating the population into using SI units in their daily lives as well as for any costs associated for businesses or consumers to complete metrication.
Canada should also remove any legality of non-SI units and prohibit them in any advertisement as well as trade. Shops for example still using pounds would be required to switch to kilograms in both sales and advertisements. Those customers who insist on asking for pounds would be sold 500 g as is done in other markets.
Back at the time of metrication in the 1970s Canadian products were being switched from rounded imperial to rounded metric sizes until NAFTA came along and products switched to rounded USC even though often labelled in its SI equivalent only. Canada needs to end this and return to rounded SI sizes. Canada needs to become the new Australia.
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The article says that, for the UK and USA, dual unit speedometers were “introduced many years before the planned conversion dates”.
I don’t believe this was true for the UK. The original 1973 conversion date was announced in 1969, and I don’t think dual unit speedometers ever became a requirement for new cars before speed limit metrication was postponed indefinitely at the end of 1970.
At that time, only a few select models were being built with dual unit speedometers installed, and these were being promoted as a feature to aid drivers when driving abroad.
Is there any evidence, from that time, that a lack of vehicles with dual unit speedometers was being used as an excuse not to adopt metric speed limits on UK roads?
However, as far as I’m aware, dual unit speedometers have since been compulsory in new UK cars from 1977, so for many years there has obviously been no way that the number of vehicles without dual unit speedometers could form any excuse for not switching speed limits to km/h.
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If I understand the history of UK law correctly:
I’d be happy if anyone can find a requirement for km/h in UK law that predates that.
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The following SI came into operation in December 1976:
https://www.legislation.gov.uk/uksi/1976/1890/contents/made
This adds the UK’s implementation of 75/443/EEC, regarding speedometers, to the schedule of The Motor Vehicles (Type Approval) Regulations 1973.
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There is also this SI:
https://www.legislation.gov.uk/uksi/1976/1507/contents/made
which adds the 75/443/EEC speedometer requirements to The Motor Vehicles (Construction and Use) Regulations 1973. This affects new vehicles from 1 January 1977, as per Article 7 of the Directive.
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Thank you for those helpful references which predate the ones I gave.
I’m struggling to understand the mismatch between the dates.
Ten years later:
Why did the later legislation use the vehicle first use date of 1 April 1984 rather than 1 January 1977?
I found it difficult to understand the various laws and can’t explain the mismatch of over 7 years. However there appears to be a grandfather clause in section 4(6) of the 1973 regulations which might explain some of it. Confusing.
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Perhaps it’s just me, but post-Brexit I’m noticing that some car manufacturers appear, at least on the surface, to have reverted to MPH only speedometers. I seem to recal seeing more than one car where the large dial was marked only in MPH, though it’s possible that the secondary digital display may be able to show km/h I do feel this stretches the spirit of the law in the wrong direction and it appears that nobody has flagged this as an issue; I woudln’t like to drive one of these outside the UK!
I also have some frustration with vehicles with digital-only systems. My car for instance, a 2017 Vauxhall (Opel!) Astra has an analogue speed display with both MPH and km/h shown, the digital part can be set to US, UK or Metric; the system includes odometer and trip computer, tyre pressures, fuel consumption, etc. and as I prefer metric I use that exclusively (though I set it back to ‘UK’ when I take if for an MOT, believe me you don’t want to go through the hoops needed to correct an MOT certificate that the examiner has put the wrong ‘mileage’ on). My motorbike on the other hand is either US, UK or Imperial; the speed is completely digital so really needs to display the proper speed limit (I recently did an IAM advanced rider course and they frown on speedos showing km/h in the UK) so I have to put up with miles on the odometer and MPG fuel consumption.
I was also surprised during a European road trip on the motorbike last year that so many motorcyclists leave their bikes set to MPH. I don’t know if this is because they don’t know it can be changed, can’t be bothered to find out, or are just too pig headed to use the right system. But I loved having 10 days without a single imperial unit in view!
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Most new cars have digital speedos. It is damn easy to flick between MPH and km/h. I use km/h in my car and also use km for navigation. It is no issue at all. Even speed limits are automatically converted so 40 MPH shown as 64 km/h in the dashboard.
If your car supports km/h speedo, then no one is stopping you to use it.
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