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?