Vehicle fuel consumption – another muddle

We look at vehicle fuel and energy consumption, and put some questions to owners of hybrid cars.

In an article posted on 30 June 2018, we asked if the pricing of domestic gas by the kWh rather than the joule makes ‘metric sense’. In this article we look at an area where little makes sense – the measurement of fuel consumption.

An American staying with me last week was heard to remark: “You buy your gas (ie petrol) in litres but you give your fuel consumption in miles per gallon. Is this practical?” I did not dare to confuse him further by pointing out that identical cars will achieve 20% more mpg in Britain than in the US as the Imperial gallon larger than the US gallon.

The official measure of fuel consumption is litres per 100 kilometres (L/100 km). This makes sense elsewhere in the world (except in the US, of course) as calculating total fuel consumption on a journey then involves multiplication not division. But it is unhelpful in the UK as our road signs remain in an Imperial time warp.

But how is fuel consumption measured for hybrid cars? Owners please advise.

One option would be to treat the car as two separate vehicles, and consider fuel and energy consumption figures for the two different modes: mpg or L/100 km for one part of the journey and kWh/100 km or MJ/100 km for the remainder. And then rely on an in-board computer to work out average and total energy consumption for the journey.

Owners of hybrid cars: is this how it works?

And alternative would be to price fuel by its energy content. Typical figures for the latter are: diesel 39 MJ/L, petrol 35 MJ/L, LPG 22 MJ/L. Imagine pulling into the petrol station and seeing petrol priced at 3.7p/MJ. Hmmm, that sounds cheaper than electricity, currently priced around 15p/kWh or 4.2p/MJ. But how efficient is the car’s petrol engine?

Owners of hybrid cars: how do you compare the costs of running on petrol and electricity?

There is one consolation. British thermal units (BTUs), therms and foot pounds are unlikely to figure in this discussion.

In our final article on the energy measurement muddle, we shall look at food labelling.

15 thoughts on “Vehicle fuel consumption – another muddle”

  1. Well, I can offer the US as a counter-example, the way not to do it. Whatever energy source the vehicle uses is converted to a mythical gasoline gallon equivalent (GGE) as the “large print” fuel economy on the Monroney sticker of a new car. (A sticker on the side window that gives Federally mandated vehicle specs, fuel economy and emissions data). NIST and the EPA seem to disagree on whether a GGE is 114 000 or 115 000 BTU, but both convert to kWh or MJ based on the BTU defined at 60 °F, and much loved by the petroleum industry. The EPA figure (1 GGE = 115 000 BTU, 33.70 kWh, 121.3 MJ) is used for the Monroney sticker and advertised fuel economy.

    In smaller print, gallons per hundred miles for gasoline phase, kilowatt hours per 100 miles for electric phase and range in each phase are given. I believe an hybrid vehicle must have at least enough battery range to complete each leg of the Federal drive cycle to have an electric rating. Wikipedia has articles on the Monroney sticker, miles per gasoline gallon equivalent, etc, if anyone wants further info.

    The way to compare alternate fuels is on an energy basis, and then pricing of that energy. For any internal combustion engine, that energy should be based on Lower Heating Value (LHV) of the fuel (Assumes energy in water vapor of exhaust is not usefully recovered). The only exception might be a PEM fuel cell using hydrogen and exhausting water at below boiling temperatures. For an electric vehicle, it should be based on recharging energy, not what is drawn from the battery, as the battery is not 100% efficient.


  2. @Ramsden

    I think you should have confused your friend further as it may have made him realise how truly muddled pre-metric units are. The entire way fuel consumption/fuel economy is delivered is designed to be a muddle. It is done the way it is in order to discourage consumers from bothering to figure out what they are really getting.

    In response to John Steele’s remark on the US Monroney sticker now displaying albeit in small print the fuel consumption rating it should be noted that only recently was it discovered or at least brought to the attention of the authorities. From this link:

    It proves that fuel consumption values are far more accurate in presenting fuel usage data. But for some odd reason, the Americans, Canadians and the English are incapable of learning the better way and cling to out dated and erroneous figures. Canada officially uses litres per 100 km as the primary unit set they also give mpg figures so no wonder the man on the street continue to monitor their fuel usage in mpg. I’m not sure if it is US or UK mpg. It has to be very complicated to convert both fuel and distance to Luddite units and then do the calculation, so I’m sure most people don’t bother.

    Fuel consumption information for Canada:

    Other English countries that metricated and don’t bother with Luddite units have seen these units disappear from common use. As long as governments continue to include Luddite units the masses will cling to them.


  3. Does anyone happen to know how fuel consumption for cars and trucks is done on the stickers and in advertising in Canada, Australia, New Zealand, and South Africa? Just curious …



    This article in the Daily Wail needs to be made a topic and the points he makes expanded upon. The points that people are using metric units freely without being told to.

    This man is literally upset because people in the UK are freely choosing to use metric measures even when they don’t have to and this bothers him. Maybe in 2001 there was a strong pro-imperial bias in the shops, but 20 years later the people are freely choosing to do business in the metric system.

    “But as with so many other aspects of our lives, they were destroyed with slow subtlety and cunning, and resistance is left to a few eccentrics, such as I am, fuming hopelessly against a loss nobody else can see.’

    This comment of his is what perfectly defines a Luddite and all forms of Ludditism must be opposed.


  5. “The usual imperial/metric mishmash.”
    This is the comment that can be made to literally any and every motoring journalist’s report.
    With battery powered passenger cars become increasingly mainstream, at some point Luddite motoring journalists will be compelled to bite the bullet and give motor power output in kilowatts (kW). Here in Japan (you know, that country with some 12 major automobile manufacturers), without exception engine/motor power output is given in kilowatts.


  6. @Daniel Jackson

    Here is a link to the Canadian fuel economy label. It emphasizes fuel consumption in L/100 km. There is a “small print” conversion to mi/gallon (based on Imperial gallon) underneath. The graphic style is similar to the fuel economy portion of a US Monroney sticker.

    Australia: no miles or gallons appear at all:


  7. @Ezra Steinberg – South Africa certainly uses litres per 100 kilometres. This unit makes sense – it is cost per unit benefit – the cost is measured in litres of fuel and the benefit in distance travelled.

    If I go to the shops, I see meat and vegetables priced at £X.XX per kilogram – again cost per unit benefit – the cost is in pounds sterling (or US dollars, euros, rands etc), the benefit is in kilograms of meat or vegetables as the case may be.

    The concept of “miles per gallon” was born when the test to evaluate a car’s fuel consumption was first evolved – one gallon of fuel was added to the fuel tank and the car driven to see how far it would go before the gallon was used up – ie mpg describes the experiment, not the quantity.


  8. BIPM SI Brochure:

    In forming products and quotients of unit symbols the normal rules of algebraic multiplication or division apply. Multiplication must be indicated by a space or a half-high (centred) dot (·), since otherwise some prefixes could be misinterpreted as a unit symbol.

    So, kWh is wrong and kW h or kW•h is correct.


  9. As I prepare for a week driving a rental car in the USA (which, when I pick it up, will have all it’s digital displays switched from US Customary to Metric), I remember several things from my past.

    The first being folk who always believed fuel consumpion on American cars was worse because the MPG figures are always lower in US adverts. I even have a very intelligent (used to work for Boeing as an engineer) friend in California who once quizzed me about why the Volvo he was looking to buy had better fuel consumption in the UK brochure.

    The second, being closely related, that even European beer purchased in the USA, never seems as strong as in the UK and you have to drink more pints to have the same intoxicating effect.

    The third was the cashier at a gas station in Orlando just a few years ago who actually said to me that he was perplexed at why the US hadn’t switched to litres.

    And so now I prepare to be bamboozled by the “Lane ends in xxx feet” signs on the interstate!!!


  10. @Alex Bailey

    Sadly, even US towns near the Canadian border (and I suspect the same near the Mexican border) do not show metric anything (even though US law and regulations permit signage in metric in addition to the US Customary). 😦


  11. @Daniel Jackson:

    But I doubt if one in a thousand now living in these parts […]

    Sounds a bit decimal—has a subeditor corrected it from the original dozenal seven gross and eleventy three? Daily outdoor temperatures at top of page exclusively in °C; sacrilege 😍. Even in 2001, shopkeepers not working in metric primary were a big red flag that you were about to be swindled—comedy antique balances with blunt knives and probably shaved counterweights, rounded-down measures and rounded-up prices, etc. Farmers in the 1970s were already impressively metric but imperial acres persisted until much later even though it made yield calculations harder. Their roadside stalls are just using what the customers want and understand. It’s probably honesty boxes, so the author might get away with taking ounces and leaving florins and farthings once. If he kept doing it, though, they’d chase him down and knock some Metric (& Decimal) Sense into him 🤣!


    Also true of motor cars made in UK by Japanese (and other) companies for decades, even if only secondary/ supplemental indication. As is often the case, the muddle is whipped up and promoted mainly by the ‘journalists’—they really are liberal arts muppets!

    @Alex Bailey:

    They’re not fobbing you off with those funny 473 ml USA [short] pints of beer, are they 😧? You’d think any real-ale brigade visitors would get their beards in a right knot about that—they want their full 500 ml of alcohol and aren’t fussed about the 68 ml–70 ml of foam on top which is approximately 99 % air anyway…


  12. @Ezra Steinberg

    Having driven near both borders in recent years I know what you mean though I do recall signs on the US side of the Canadian border in upstate NY having information signs about speed limit differences but little more.

    California seems to have made a token effort sometime in the past, several times I’ve passed a dual mi/km distance sign southbound on the I-10 going from LA to San Diego but I was actually shocked last Saturday on the way back to LA from Las Vegas to see a similar sign southbound on the I-15 just north of San Fernando, a lot further from Mexico that the other one!


  13. I happened to come across an interesting article on badging new Cadillacs (cars), for the US market, enter the Newton Metre. : – Typically, American car makers rate their torque in lb-ft, the newton meter rating is what is found globally however, since it’s based on the metric system. The new badges will appear on the all-new XT6 crossover.


  14. Brian,

    Torque units are units found mostly in industrial applications are rarely encountered by people on the street. With that in mind the units used don’t follow the convention of the country, but the convention of the industry. The American and International Automotive Industry has been metric since the 1970s and thus the newton-metre is the typical and standard unit used in this industry. So it would be incorrect to claim that the Automotive makers use pound-feet. They in fact internally do use newton-metres.

    Before the metrication efforts of the 1970s, US industry was virtually 100 % USC, but since the metrication efforts US industry is a muddle and it can’t automatically be assumed that all US industry is USC. Some companies are metric, some are USC and others are a mix. The claim that the metrication efforts failed is somewhat true but only as far as what the man on the street uses. In many examples metrication was a success, such as in wine/spirits, automotive, medicine, electronics, science, much of engineering, etc.

    It’s a muddle of the worse kind imaginable and the US is paying the price. China and other countries are passing the US by as a result.


  15. Daniel

    Yes, you are correct of course. The writer of the article mentioned lb-ft to make a point I guess, as that is what goes public.
    The good point is that Cadillac are going public with Nm on the badges of their cars, that must be great (terrific even) news for metric advocates.
    For the not so metric minded it removes the confusion of lb-ft and lb-in, I have been confused on that one myself recently.
    The point of my post was that Cadillac are doing this is order to make figures more international (for the China market?), and it may just catch on, next step is for engine power in kW going public.
    Using real figures may help sorting out the strange computations we have to do to understand things at present.


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