Our awkward vehicle specifications

Ronnie Cohen looks at some consequences for the motorist of the UK’s measurement muddle.

Have you ever considered buying a car in the UK? If you search for a British car, you are bound to be exposed to typically and uniquely British car specifications. In case you are thinking, “Ah, the Americans use the same or similar car specifications” remember that the Americans use a different gallon, which is a lot smaller than ours, so our miles per gallon fuel economy measures will be very different from the American equivalent, even for the same car!

The relationship between British car specifications and the units used in British road signs is an awkward one. This makes some conversions between imperial and metric units inevitable. The conversion factors are not round numbers so if you want to work out some practical measurements, you will either need to be good at mental arithmetic or you will need a calculator.

Vehicle dimensions in car specifications and car manuals are expressed in metric units. While the TSRGD 2016 made metric units on vehicle dimension signs mandatory for the first time, existing imperial-only versions can remain in place until they wear out and need replacement. There are still many that only show feet and inches. Now, if you are buying a van, over 2.2 metres in width and there is a maximum width limit sign with a measurement of 6 feet 6 inches on the road to your house, how easily can you work out if your vehicle will pass through the bollards on each side? Or if you are driving a lorry that is 2.9 metres tall and you see a maximum height limit sign of 9 feet 3 inches, can you work out if you will hit the bridge?

When you drive on a motorway, you have probably seen marker posts on the sides of motorway with numbers that increase or decrease in increments of one tenth. Have you ever wondered why your odometer does not change at the same rate as the marker posts as you go past each one? No, there is nothing wrong with your odometer! The marker posts show kilometres while your odometer shows miles.

The speedometer in your car shows both miles and kilometres. This is a legal requirement. The legal requirement to show kilometres was introduced in preparation for Britain’s changeover to kilometres on British roads, which has not happened yet. While Britain retains miles on its road traffic signs, we are stuck with a dual display of miles and kilometres on our speedometers.

Car CO2 emissions are given in grams per kilometre. However, long road distances are only shown in miles. So how many grams of CO2 emissions will be caused by travelling from London to Oxford, a distance of 60 miles? When you see distances in miles and fractions of a mile, would you know how to work out emissions?

With the aim of reducing emissions, governments are encouraging the switch to electric and hybrid cars. So how should their power be expressed: horsepower (a unit proposed by James Watt around 250 years ago when horses where the main source of motive power) or the kilowatt (named after you-know-who and a universal unit for electrical power, understood by anyone who owns an electric kettle)? And how should fuel economy be measured to enable the performance of all types of car – petrol, diesel, hybrid and electric – to be compared?

Fuel economy figures for petrol and diesel cars are expressed in miles per gallon and in litres per 100 kilometres. Both fuel economy measures are awkward when fuel sold by the litre and long road distances are only expressed in miles. For example, suppose that you want to drive from London to Manchester, a distance of 210 miles and you fuel consumption is 50 miles per gallon. Do you know how to work out how many litres of fuel you will need for your journey? Or suppose that your fuel consumption is 5 litres per 100 kilometres. Do you know how to use that to work out how much fuel you will need to drive 210 miles?

I also wonder if many young drivers know how many litres there are in a UK gallon. Fuel ceased to be sold by the gallon in the mid-1980’s, so it is a mystery why we still use gallons for fuel economy measures. And how many British drivers would even attempt such calculations. You can only do these calculations if you know the relevant conversion factors and how to do the calculations, whether mentally or with a calculator. It would be a lot easier if we just used metric units, which are simple, straightforward and all based on powers of 10. But you know that already. How long before our politicians and the UK Department for Transport admit that they accept it too?

This entry was posted in Consumer affairs, Transport and tagged , , , . Bookmark the permalink.

9 Responses to Our awkward vehicle specifications

  1. Cliff says:

    Two bridge strikes reported in the UK news today where buses have lost their top deck.
    Road signs in both cases were imperial only

  2. Jake says:


    I'm not sure that the bridge height signs were the issue here. If a bus driver, or drivers, see a low bridge and are aware that they are driving a double decker bus, they must surely realise that the bus will not pass beneath the bridge simply by looking at the bridge. If it looks too low, it probably is too low. I believe there is more to these stories than has been reported in the press. This is surely a matter of training. I would hazard a guess that the drivers have public vehicle licences but do not necessarily have training on double decker buses. (Is that possible?) Thankfully the accidents were no worse than they were.

  3. Daniel Jackson says:


    You provided a dead link. I tried it a number times throughout the day with the same results.

    Page not available

    Unfortunately, we are unable to bring you the page you are looking for.

    We may be suffering from technical difficulties, or you may have typed the web address incorrectly. Please check the address including its spelling, and try again.

    [Thanks. WordPress conjoined the links - now corrected. Ed.]

  4. Cliff says:

    Sorry Daniel. I copied the links in a hurry and hadn't realised that they hadn't pasted properly.
    [I think they were correct but got scrambled by WordPress! Ed.]

  5. Daniel Jackson says:


    The bridge height restriction sign was imperial only in the first video and metric-imperial in the 2nd. The height was stated as 3.6 m.

  6. Cliff says:

    Look carefully at the video again. The height restriction sign on one side of the bridge (where you can see the front of the bus) is both metric and imperial, but the wonky sign on the side of the bridge that the bus approached on (and probably hit) is imperial only. A look at Google Street View or other news sources will confirm this.

  7. Jake says:


    I water down slightly what I said about the bridge height signs not being the culprit. I indeed noticed a dual sign in the video clip, but you say the bus approached from the other side which had an imperial-only sign. While I am sure that bus driver training is also an issue, it does beg the question of why the local council signs on one side of the bridge are in 'old money' only and the other side is in metric too, the latter being what a driver today will have learnt at school. While the change in the law to make metric bridge height signs mandatory for new signs is a step in the right direction, it does rather make me wonder how the driving public are expected to cope with signs in two different systems of measurement at the same time until such time as the imperial signs are worn out, which could take years. Surely all drivers were taught metric today and those who weren't (and weren't prepared to adapt) must have retired by now. It is time for all bridge height restriction signs to be metric-only.

  8. Cliff says:

    I saw a newspaper report recently about a crime incident that occurred on a road called Commonside East in Mitcham, South London. The picture of the scene showed a road traffic sign with the width limit shown in Imperial measurements only (7'0"). Next to the width limit was a weight restriction sign in tonnes. About 100m further down the road was another written sign that that said: Width limit 7'0" 130 yards ahead. Apart from the unnecessary clutter, the mix of units and the absence of metric distances is ludicrous.
    Drivers under the age of 40 or 50 are not fully consignant of imperial measurements and drivers from abroad must be really nonplussed. These signs are inviting an accident to happen. Whoever authorised them should be doing another job.

  9. Daniel Jackson says:


    The BWMA is probably behind this. They don't want metric on the signs because in their fantasy, if metric is not present, the driver, both foreign and domestic will make an attempt to learn imperial and imperial continues on for another generation. But, they are too thick skinned to realise, that people will just ignore the sign. At least with the sign in yards, they can envision it as 130 m by assuming, right or wrong, that 130 yards and 130 m are the same thing. But, the feet inch dribble will just be ignored.

    It would be terrible if a driver thought the 7' was meant to be 7 m and a driver with a wide load in excess of 2 m width would attempt to pass through.

    The only way to prevent an accident would be if everyone has a GPS system and they set it to metric mode and GPS voice warns the driver of the width in metres despite the sign.


Leave a Reply

Your email address will not be published. Required fields are marked *