An article published on the Guardian web site on 20 June describes US research on the public perception of vehicle fuel efficiency in relation to the units used.
A reader of Metric Views has written to us as follows :
“I read this story on the guardian website today and thought it may be of interest to you:
http://www.guardian.co.uk/environment/2008/jun/20/travelandtransport.carbonemissions
To summarise it, it has been proposed the fuel efficiency in the UK and the US is displayed as gallons consumed per so many miles rather than the number of miles that can travelled per gallon (gpm rather than mpg).
The case for this is valid – it shows the potential fuel savings of a more efficient vehicle more clearly. However there is no mention of the fact that this is how metric countries have been measuring fuel efficiency for many years (as litres per 100 km).
If they are going to change the system it would make sense to adopt the international system.
Admittedly there is still the issue that we use miles for everything so the public would understand gallons per miles better, though perhaps litres per 100 miles might be a compromise moving us in the right direction?”
Metric Views wonders what are the chances of the USÂ changing the habits of a century. And we are not sure about the suggestion of litres per 100 miles – temporary arrangements have a habit of becoming permanent.
The web site does not appear to have facilities for comment on this story, but it may be possible to respond to Ian Sample, the Guardian science correspondent.
(And don’t forget whichever way round you put the numbers, there are 4.546 litres to an imperial gallon and only 3.785 litres to a US gallon.)
Metric Views 
I get the feeling that many British drivers actually work out the figure as “miles per metric gallon” (meaning 1 gallon = 5 litres) since the maths is clearly easier and most drivers under 30 and quite a few under 40 probably don’t ever remember buying petrol in gallons anyway! This would clearly mean that figures given by manufacturers in new car brochures and other advertising bear little resemblance to what drivers actually experience.
I switched to using L/100 km a few years ago and merely multiply the distance on my odometer by 1.6, I’ve found it much easier to calculate fuel costs and have a much better handle of how far I can drive on any given quantity shown on my fuel gauge than I ever did using mpg.
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Quote: Admittedly there is still the issue that we use miles for everything so the public would understand gallons per miles better, though perhaps litres per 100 miles might be a compromise moving us in the right direction?�
Litres per 100 miles would be a bad compromise. It means getting use to one set of numbers then when road sign metrication finally happens having to get use to a new set in short time. The better compromise would be to call it litres per 62 miles, at least this way the numbers are the same as litres per 100 km.
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I’ve never bought fuel in gallons. In fact, I’ve never bought anything in gallons and never seen anything for sale in gallons. I can only use mpg to see how more fuel efficient my car is compared to other cars, I’ve never used it to see how much fuel I would use on a particular journey.
Introducing “gallons per mile” would be as much use to me as mpg is. Now L/100km – that is useful, and I can use it to quickly calculate how much fuel I’m likely to need for a particular journey. It would also make me think twice about making a journey by car. Shame the information displayed in my car can’t help me with this.
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It’s not a case of litres/100 miles being easier to understand than litres/100 km it has more to do with not knowing distances in km in the UK.
Motorists are forced to think in miles because of road signs, road atlases and car instrumentation as well as most other sources of travel information.
We cannot expect the average UK motorist to make the mental effort it takes to think in km in those circumstances.
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Once again we see the integrated nature of the problem and how the anachronism of Imperial road signage continues to serve as a key road block (pun intended) to completing metrication where it counts: in the daily mental activity of the citizenry.
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I keep my car’s digital computers in metric, as, having lived in Canada so many years, L/100 km means much more to me. That is what people there ONLY use these days, although was a time when some people did a double conversion to miles per (imperial) gallon, notwithstanding having bought their fuel in liters and their car recording the distance travelled in km. Those days are now long gone, and people there only understand L/100 km. Today, even in imperial UK, I find L/100 km much more useful and meaningful, and mentally convert all my car journeys into km.
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