How to convert fuel consumption figures (if you must)

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)

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16 Responses to How to convert fuel consumption figures (if you must)

  1. Sean Wisehall says:

    You give conversion formulas like theres a standard size. Outdated units are defined any which way. Most of them are just names for rounded metric quantities. The conversion factor could be 235 or 200. Either one is just as valid when you use unsupported units.

    The sooner consumers ignore the imperial noise the easier the transisition. Conversions are strongly discouraged, as they are simply a temporary crutch. People can adapt quickly. Regardless of what fringe anti-EU groups say, I have no reason to believe the British are an exception to this.

  2. Dave Brown says:

    I always work with L/100 km for fuel consumption. It makes calculating a journey cost far easier. If you know the distance you are travelling as a number of km (one mouse-click on a route planning website will give you this), then divide by 100 (a trivial calculation) and multiply by your L/100 km figure for your vehicle (a simple multiplication). This tells you the number of litres of petrol required, and your local petrol station tells you how much each litre costs.

    Contrast that with the convoluted calculation to get from mpg to cost of journey (involving a division sum followed by a conversion from gallons to litres). I can't understand why people still cling to this relic of the 1970s which is mpg.

  3. Phil Hall says:

    Why do people in the UK continue to measure in mpg even though we no longer buy fuel in obsolete gallons? Even though mpg uses the same distance unit for car journeys, (unnecessarily forced on the motorist for more than 30 years) it still doesn't avoid the awkward arithmetic of conversion (litres to gallons).

    I think it could be a legacy from the days before self-service petrol pumps and the predominance of cash transactions. It was more convenient then to ask for say "five quids" worth of petrol so that you could hand over the £5 note and not have to wait for change. This meant it was more natural to think of it in terms of how far you'd get for your £5 rather than how many gallons it would actually take to meet your travel needs (note that mpg is the inverse of fuel consumption).

    If we were to change over to the metre/kilometre for road signs then and only then will the motorist (rather than just the petrol retailer who prefers smaller units for trade) finally benefit from the change to litres. It would then become perfectly natural and more convenient to measure in L/100 km. As it is we benefit from neither.

  4. Alex Bailey says:

    While I agree that there should be no need to use MPG in this day and age, unfortunately it is still sometimes still necessary. Earlier this year while trying to rent a car for a trip to Canada I came up against the Hertz web site which quoted fuel consumption on my chosen vehicle in MPG (I could only assume this meant US gallon though).

    More recently I've found fuel price reports published by the AA which show the price of petrol and diesel in gallons.

    I only switched to using L/100km about 2 years ago and have found it much easier to use... but while we're still forced to buy cars which only measure distance in miles we're going to be stuck with MPH because when we switched to buying fuel by the litre it was (and still is) the custom to look at conversion charts and convert back to gallons, hence it will seem easier to most people to do than to convert miles to km.

  5. Ezra Steinberg says:

    The preceding comments make it abundantly clear (yet again) how much of a ripple effect the changing of road signs to metric will have on finally finishing metrication in the UK (leaving aside pints of beer and milk). I think this is why anti-metrication folks are so adamant about retaining Imperial road signs. Everyone understands at some level that converting the signs will sweep away most vestiges of Imperial in fairly short order.

    I'm wondering if British trade and manufacturing associations have been (or can be) persuaded that the competitiveness of the British work force and school education in science and maths can be enhanced by removing the muddle that still prevails. Perhaps that is the most effective route to effect a change absent the political will on the part of the government to do so.

    I would hope that a gradual changeover using metric signs with Imperial overlays would allay the concerns some have about the cost of the changeover. One could even hope for an M-day for road signs just ahead of the London Olympics!

  6. Crooked Miles says:

    The use of mpg has been further entrenched by in-car computers. I own a 6 year old Skoda Fabia, which, at the flick of a switch, gives information such as temperature (deg C only), miles travelled, average consumption (mpg only). There is no way of altering the default settings on this instrument.

    Really, it is very hard to make the case to dump mpg in favour of l/100km while distances are measured in miles - it would mean the exchange of one partially-fitting system for another. L/100km seems to be becoming the norm in Ireland since their speed limit changeover in 2005, so that would presumably happen here in similar circumstances.

    Conversely, I see no excuse whatsoever for continuing to quote tyre pressures in PSI in the UK.

  7. Sean Weisthal says:

    "L/100km seems to be becoming the norm in Ireland since their speed limit changeover in 2005"

    I wonder if this is reflected in whether people use mpg or l/100km, ie - if you say to someone in Ireland "What economy can you get out of your car?" what would their natural response be....

  8. Alex Bailey says:

    I agree with Crooked Miles that in-car computers are not helping. In my experience, North American cars with such computers give a clear choice, metric or customary - since they make the same vehicle for both US and Canadian markets it makes sense as you have less to change (different analogue speedo and that's it).

    There seems to be no similarity in Europe though. Even though Ireland has now gone metric it seems that motor manufacturers would rather just do one or the other and not both. I recently got to play with a Seat Ibeza which had a similar trip computer which the owners manual demonstrated in metric but the one on the car was fully imperial with no obvious way to change it. Similarly with the digital speedo on a friend's Citroen C3 which although, in compliance with EU law, can be switched to metric, had no instructions to do so in the owners manual!

    One saving grace is that I have recently found a package for my Palm PDA which allows me to enter fuel usage in litres and distance in miles to give me fuel consumption in L/100km!

  9. Ezra Steinberg says:

    Crooked Miles wrote: "Conversely, I see no excuse whatsoever for continuing to quote tyre pressures in PSI in the UK."

    Living in the States, I had no idea the UK was still using PSI for tire pressures. Forgive my ignorance, but how could this be? I thought pressure (such as barometric pressure in weather reports and other scenarios) was routinely given in kilopascals (or worst case bars or millibars) these days in the UK.

    What's the story behind this, I wonder? And is the situation the same in the Irish Republic?

  10. George Carty says:

    Kilopascals? Are hectopascals not better for backward-compatibility with millibars?

  11. David says:

    Ouch. All this talk of formulas seems very complicated, and is probably beyond the mathematical abilities of many people. That's why people prefer, in their "ignorance" (in the sense that you "don't need to know"), just to have a number presented to them, and all that they need to know is whether a larger or a smaller number is better.

    Unless I'm missing something, surely the only time one needs to know the fuel efficiency of a car is when choosing a new car to buy, and unless I'm mistaken, it seems that all car adverts include the fuel consumption in L/100 km (or mi/gal) in the small print in any case?

  12. Martin Vlietstra says:

    As David rightly points out, not everybody is technically inclined.

    Maybe I should have started off by saying that a bigger number for the price of apples indicates "more expensive". If we express fuel consumption in L/100km, then likewise, a bigger number means more exopensive. However, when using mpg, we have the perverse situation of a smaller number meaning "more expensive".

  13. David says:

    Ah, right. I see what you mean now, Martin. Just as more expensive apples cost more money (per kilo), for fuel it is the same: the more gas-guzzling the car, the greater the cost in terms of fuel used, hence the number of litres used/100 km is also a bigger number.

    I still reckon that for the average person, it's probably still easier to look up the figures in manufacturer documentation than try to convert, mind you.

  14. Shaun says:

    PSI makes more sense to me than "bar" and "pascals".
    If you must have a metric version then why not Kg/cm^2? Something one can easily envisage.

  15. Ralf says:

    Throughout my upbringing and university eduction in engineering in a metric country it was safe to assume that 1 bar equals 1 kg per centimeter square.

    The pressures one commonly encounters are usually single figures in bar and unless you are an engineer or service personnel for high pressure systems you will never have a problem with the 1.196% difference between the two units of measure.

    Even then no high pressure system should be designed for safety reasons so close to a critical pressure value that it matters if 1 kg/cm2 is only 0.9804 bar and not 1 bar.

    Your typical car tyre is somewhere between 2-3 bar depending on the size of the tyre in relation to the weight of the vehicle.

  16. Brian Capon says:

    I never realised the extent of this problem until I sent an email to my local MP after I had bought a car tool for which I had to try to get American (UNC) bolts to make it work. One should be able to assume all bolts in UK would be metric. Now to this new issue.
    The media perpetuate these strange units as much as anyone. Top Gear and even the pinnical of technology formula one, still uses all the odd-ball measurements, PSI, MPG, Hp despite broadcasting to much of the world that have no idea what they mean. The weather forcast always give degrees Farenheight even though I can niether spell it nor know what it means any more, rainfall in inches. bodywieght (humans and other animals) in stone, that I have never understood. Then we have 'swimming pools' 'football fields' 'golf balls' and all manner of wierd measurements. What I want to know is as I am in my 68th year and very much of the lesser educated group, who are these people that 'don't understand' or 'can't adapt' to being fully metricated. Do these presumptuous presenters think they are helping the likes of me? I find duplicity, indeed multiplicity by far the most difficult problem to overcome.


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