UKMA discourages conversion between metric and imperial units. Much better to understand and use metric only – and forget about imperial. Occasionally, however, it is necessary – e.g. when trying to understand historical data or American stories. The following technical advice is offered by Martin Vlietstra.
The conversion of fuel consumption in imperial units to metric units is not as straight forward as other conversions. The conversion formula can be written as:
m·i = 282
m is fuel consumption in L/100 km (metric units)
i is fuel consumption in mpg (imperial units)
One of the implications of this formula is that 16.81 mpg = 16.81 L/100 km [because 16.81 is the square root of 282 – Editor]. Also, if you halve one of the values, then you double the other. Therefore
8.40 mpg = 33.62 L/100 km and
8.40 L/100 km = 33.62 mpg.
The underlying reason for this peculiarity is that mpg does not follow the norm; it is equivalent to saying that you can buy 2.5 kg of apples for £1 rather than saying that apples cost 40 p/kg. Expressing fuel consumption in litres per 100 km expresses a cost in the standard way, expect that we are using litres of fuel rather than money as our currency.
(Readers who are more familiar with US units should use the relationship m·c = 235 where c is the fuel consumption is customary units)