Prompted by a letter in this month's RIDE magazine I've been wondering.
Shouldn't we ditch MPG and quote the figures in litres per mile - after all we know how much a litre of fuel costs - most, including me, would struggle to do an on the hoof conversion to cost per gallon. The guy in the letter says his bike does 10 miles per litre....makes sense to me, seeing as we use lites and miles in this potty country.
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PU, even off the top of my head I know that 10 miles per litre = 45mpg (approx). It would be more sensible, though I fear we'd be dragged into the bonkers litres/100km measure. It would also "compress" the numbers into a smaller range thus flattering somewhat the thirstier cars. And then there's the devil you know of course :-)
John
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Just wait until Martin see's this he'll be spitting nails !
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Works in rods and perches doesn't he? With the odd furlong thrown in. Oh and you buy nails by the pound :-)
John
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>> Works in rods and perches doesn't he? With the odd furlong thrown in. Oh and
>> you buy nails by the pound :-)
>> John
>>
Can't buy them with a Pound any more. Phartoo expensive...0:)
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The use of litres instead of gallons was a nice easy way to disguise the true cost of a gallon of fuel and of course a couple of pence per litre here and there usually goes without much fuss.
I filled my Fazer up yesterday and was quite startled to find I spent £28. it wasn't so long ago that it was £20.
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>> The use of litres instead of gallons was a nice easy way to disguise the
>> true cost of a gallon of fuel and of course a couple of pence per
>> litre here and there usually goes without much fuss.
>>
>> I filled my Fazer up yesterday and was quite startled to find I spent £28.
>> it wasn't so long ago that it was £20.
>>
Without doing the sums that seems outrageous for a bike. We are all DOOOMED.
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The computer in my Mazda quotes consumption in litres/100 miles. If I remember correctly 11.7 translates to just about 40 mpg but I have to carry a ready reckoner in the car to give me a vague idea of what I'm using.
The reckoner was provided by a correspondent on another site (could have been 'number cruncher') when the subject came up a couple of years ago.
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I've reset the BMW to litres / 100km because it's slightly more soothing looking at smaller numbers when it comes to fuel consumption.
Usefully you can also view it in litres per hour, which led to to work out that if I forgot to switch the engine off on a full tank it'd run for nearly a week.
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Back in 2004 when Diesel was around the 80-odd pence / litre mark, i could fill the wee corsa from the start of the red to brim full for ~£28.
Today that would cost almost exactly double.
Probably would be doing less miles to a tank by now too :-(
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>>into the bonkers litres/100km measure.
Although I'm rabidly anti European, the litres per 100 km measure is actually extremely sensible.
There's a latent problem with using mpg as a measure - it's non-linear - going from, say 10 mpg to 11 mpg represents a much bigger saving than, say, going from 49 to 50 mpg.
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No we don't need to change, and nothing at all wrong with gallons as a measurement either.
If that's ok with our eu masters.
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>> No we don't need to change, and nothing at all wrong with gallons as a
>> measurement either.
>>
>> If that's ok with our eu masters.
>>
Stuff the EU masters. I don't care how much more "sensible" litres/100km might be, I've worked in mpg all my life and it's good enough for me.
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The problem is that a change of, say, 1 mpg doesn't represent a consistent change - you need to do a few more calculations to see the true effect.
If you try the example mpgs I gave, and work out how much fuel is burnt per 1000 miles, you'll find the fuel saved by going from 9 to 10 mpg is much greater than the fuel saving between 49 and 50 mpg.
mpg, as a scale is simply non-linear, and hence useless. If we used gallons per 100 miles, that would solve the linearity problem - we wouldn't necessarily need to change to use litres.
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>> If you try the example mpgs I gave, and work out how much fuel is
>> burnt per 1000 miles, you'll find the fuel saved by going from 9 to 10
>> mpg is much greater than the fuel saving between 49 and 50 mpg.
>>
I wouldn't argue with that, found that out for myself when I replaced the old carburettor on the pick-up and improved average consumption from 9 to 15 mpg!
Unfortunately, MPG is what we're used to, and whilst I don't bother my head with the intricacies of coaxing an extra 0.1 mpg out of any of my vehicles, or logging every drop of petrol (almost as anal as trainspotting IMO) I do like to be able to work out our fuel stops when out on the bikes, especially in more remote areas, and this is where mpg helps me.
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I agree with NC on this, Mpg is not linear, it distorts our understand of fuel economy figures.
It makes people think that going from a car that does 40mpg to one that does 50mpg is a big deal in the same way going from 20mpg to 30mpg is. But it's not the same at all.
12k miles at 20mpg = 600 gallons
12k miles at 30mpg = 400 gallons
Which is fine, and shows what a big saving it is from 20->30mpg...
However:
12k miles at 40mpg = 300 gallons
12k miles at 50mpg = 240 gallons
Which shows how little you save going from 40->50mpg. With so many people overlooking perfectly good petrol engines that can do 40mpg in favour of diesels that might do 50mpg (on more expensive fuel...) is not that sensible. I would say that using Mpg confuses and leads to possibly poor car buying decisions.
Put that into L/100Km and you have 14.1, 9.4, 7.0 & 5.6 respectively, which actually shows the true improvement in economy at each level, seen in the amount of fuel used.
However, although L/100Km makes more sense, I still can't bring myself to have my trip computer set to anything but mpg! :)
Last edited by: oilburner on Mon 14 Feb 11 at 11:27
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The easiest way to work out the improvement is to use percentages worked out from the mpg figures... Thats how I've always concluded that whilst diesel is only about 3% higher in price than petrol (3/4p) the %ge gain I will get is 25% based on a petrol car doing 40mpg to a diesel doing 50mpg... Works fine on that basis...
What I hate, though, is working out how many gallons I've put in the tank when the pump is in litres!
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I think that's the point though isn't it Hobby? You shouldn't have to work out the percentage, in L/100Km the difference is obvious because it's linear. A 25% saving is good, but I suspect if you ask your average non-mathematically inclined person how much you save going from 40->50mpg, they would say it's the same as 20->30mpg, or worse yet not even be able to answer the question!!
Plus you need to factor in the cost of fuel as well as the economy saving, your 25% is before the difference in fuel cost, I think? I reckon the figure should be about 21% for 4p a litre more, which for your average driver is about £300 savings a year. That's fine, but how much extra does the diesel version of the car cost to buy and maintain...another story entirely, of course... :)
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That's fine, but how much extra does the diesel
>> version of the car cost to buy and maintain...another story entirely, of course... :)
>>
Re the l/100km, yes i see your point, but the %ge is easy enough to work out, and, I suppose I'm used to miles!! That probaly applies to most people I suspect... hence many people will still convert their litres bought into gallons... and then work out the mpg!
Re the above, nothing! Its the Motability car... only savings i can make other than whether to make an "up front" payment is on fuel... So economy is key... At least for me!
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>>litres per 100 km measure is actually extremely sensible>>
I wouldn't mind miles per litre as an alternative to miles per gallon.
Though I prefer to know how far I can travel on a specific volume than to know how much I will use to travel an arbitary distance.
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How about, and this is a bit revolutionary, so hang onto something, selling fuel in gallons? since that is the problem that PU is trying to overcome?
John
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>> , selling fuel in gallons?
You vill also be on ze list...don't tell 'em Pike.
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A gallon is equal to 4.546 litres.
Petrol or diesel is still a lot, lot cheaper than my Guinness - sold by the pint (or half)...:-)
Same for Chilean red (sold in cls)...:-(
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>> Petrol or diesel is still a lot, lot cheaper than my Guinness - sold by
>> the pint (or half)...:-)
I wonder what the tax rate difference is per pint or gallon, comparing fuel to booze.
Where are we at the moment, roughly £3.60 or is it more being the tax on a gallon of fuel?
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I can think (that is visulise) in litres, gallons, yards, miles, metres, kilometres, but no way can i get my head around miles per litre.
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I suppose I don't really mind whether the standard measuring system remains or becomes something else. We are all used to mpg I guess but like anything a new system would become as well understood in time.
For myself, I've always thought of the fuel consumption of my vehicles in terms of pence per mile rather than mpg. I've always worked it out both for the purposes of claiming expenses on company cars and monitoring the running costs of private ones.
Couple of years ago it was 11/12p per mile for me and now it's 16/17p. Looked at in percentage terms it's quite a huge difference.
Or to put it another way, at that time it used to cost me around £45 in fuel to drive to London and back in a day. Now the fuel cost is about £60.
Last edited by: Humph D'Bout on Sun 13 Feb 11 at 21:45
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All cars sold in UK include both MPG and L/100 km option (if they have fuel computer).
Cars sold in some other parts of the world have these and/or "km/L" option.
I think all cars should include all options like MPG, L/100 km, km/L, mile/L etc.
Then it is just up to you which unit you prefer.
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>> There's a latent problem with using mpg as a measure - it's non-linear - going
>> from, say 10 mpg to 11 mpg represents a much bigger saving than, say, going
>> from 49 to 50 mpg.
Not is not.
If you have ten gallons, you go 10 miles further on both. Thats perfectly linear.
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I see it as a ten percent improvement at 10 to 11mpg and a .5 per cent improvement between 49 and 50
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If your bothered by percentage increases you should have bought a car that does better than 10-11 mpg.
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I like mpg because I prefer to think of it in terms of how far ill get for every gallon I put in my car. if I put a gallon in my mums car, ill maybe make it to Oakham, put it in my car, ill get to Newark. Works for me.
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dont be worried about all this gallon to metric stuff
go to a garage near you stick £5 of unleaded in your see through can
and weep
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I don't care whether we use miles-per-gallon, litres-per-kilometer, liters-per-gallon, or inches-per-litre or anything else you can dream up.
All I want is the official fuel consumption figures to bear some semblance to reality. The official testing is a complete joke, with all sorts of tricks used to give fuel consumption figures which will never stand comparison with real world driving.
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>> All I want is the official fuel consumption figures to bear some semblance to reality.>>
Never a truer word.....
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i think i just heard the first cuckoo
:-)
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I use a scanguage2, and you can choose mpg, mpl etc. I settled on miles per litre as the best way of measuring what I do - buy in ltires, travel in miles.
It also tells you the fuel a journey has just used. So I know that a run down to the station starting from cold and allowing for a little traffic, takes just under a litre. So about £1.35. The company will pay me just 32p for the trip (supposedly to cover the fuel)
Last edited by: Tigger on Mon 14 Feb 11 at 07:52
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...All I want is the official fuel consumption figures to bear some semblance to reality...
I get within an mpg or two of the combined figure, so the official figures work for me.
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>> I get within an mpg or two of the combined figure, so the official figures
>> work for me.
And me... They have to come up with some sort of comparison, the old 3 tier one was ok, but no more or less accurate as this one... At least you have some idea what to expect... Just out of interest what would people replace it with? There are so many variables in real life, that I doubt anyone could come up with anything better?
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Wouldn't miles per pound make most sense? That would then give a consistent measure across all fuels - petrol, diesel, LPG, or electricity.
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Looking to the future we should be measuring miles per unit of energy consumed. A gallon of petrol holds a certain amount of energy, as does a gallon of diesel or LPG or or a unit of electricity. Are we into Joules here? So if we measure miles per Joule you can compare across fuel types.
(Stands back and waits for explosion!).
John
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I often wonder why people look at combined figures and think its something someones measured. As I understand it, its just an average between two figures ( urban and extra urban ) that have been measured.
Problem is its subject to so many factors many of which the individual driver contributes to, so how they can ever throw out a wholly accurate figure I dont know.
Ive managed to beat my Combined figure on most cars without going silly.
I usually look at the Urban figure as a guide as thats the worst case scenario.
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I think it's only use is for comparing one car against another and only then as a guide, not as some abolute truth. Don't people notice that they get different mpg at different times of year / on different journeys and realise that it is only an indication? That said I dislike the way cars are engineered to fit around / slip under limits to the overall detriment of their performance.
John
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Anyone who reads an average fuel consumption as a realistic target without considering their own personal driving circumstances is as best naive and at worst an ideal target for con artists (or artistes to avoid discrimination legislation)
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>> All I want is the official fuel consumption figures to bear some semblance to reality.
>> The official testing is a complete joke, with all sorts of tricks used to give
>> fuel consumption figures which will never stand comparison with real world driving.
>>
Me too.
I couldn't believe it when I heard that the 'official' fuel consumption figures are all done on a rolling road and take no account of aerodynamics. How on earth are we supposed to drive in the real world without aerodynamics having an impact?
No wonder the motorway consumption figures are pie in the sky.
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A wee anecdote here if I may - I recently drove from Reading to Carmarthen and back, and decided to keep an eye on the trip computer's average MPG figure.
I drove the outbound journey in a near hurricane, heading in to the wind the whole way, and the computer showed 30mpg at the end.
On the journey back, there was a lighter wind, which was mostly at my tail, and the computer showed 37 mpg.
Both legs were done at the same indicated speed, mostly on cruise. The first leg contained no hold ups, the return leg had a half hour hold up on the M4 at Port Talbot whilst the local Constabulary tried to contain a coachload of Cardiff fans on their way back from Swansea, whose coach had broken down on the motorway.
The computer usually gives me over 40mpg for motorway journies, so the wind on that first leg had a spectacular effect.
Vehicle: 2003 Forg Galaxy 1.9Tdi auto.
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I heard that they run the car to a set speed on an outside test track and then let it coast to a halt in neutral. They use the distance it took to come to a halt to calculate the aerodynamics and rolling resistance, which they use to calibrate the resistance on the rolling road. thus giving a real-world approximation.
That's what I heard, no idea if it's true, but it sounds quite reasonable.
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>> I heard that they run the car to a set speed on an outside test
>> track and then let it coast to a halt in neutral. They use the distance
>> it took to come to a halt to calculate the aerodynamics and rolling resistance, which
>> they use to calibrate the resistance on the rolling road. thus giving a real-world approximation.
>>
>> That's what I heard, no idea if it's true, but it sounds quite reasonable.
>>
It's called coastdown testing. It's quite an involved measurement, and to be taken serously you do need the data from a large number of runs to allow ambient effects to be averaged out.
When done properly, the data from a coastdown test allow you to fit a drag power versus road speed curve to the car, and from that work out the individual contributions from aerodynamic drag, speed dependent rolling resistance, and static rolling resistance.
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>> No wonder the motorway consumption figures are pie in the sky.
>>
I didn't know there were any... The last one was the old method where they gave it at a constant 75mph? Alll we have now is Urban and Extra-Urban which is just a mix of driving...
I agree re aerodynamics, though... The Roomster is considerably less efficient at Autobahn speeds than the Fabia Estate, despite same engine and similar weights... I may well go back to an estate next time, though I will miss the practicality of the Roomster...
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>>and take no account of aerodynamics.
That's nonsense.
As written elsewhere, aerodynamic resistance is measured, and added in as a load at the wheels on the dynamometer.
However, what is true is that mpg is an important number for sales AND the methods of measuring it are rigidly standardised, anyone with half a brain can soon work out how to optimise the candidate homologation vehicle's performance on the day of the test.
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I don't think this one will be answered.
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It doesn't make sense. The idea of using litres (which word many people can't even spell) per mile, or even kilometre (which most people can't even pronounce), or gallons, is still far too complicated.
The best way to do this would be to specify a simple number that even simple people can understand (so it should be less than (say) 100).
One would then say "My vehicle does 10 fuel units" (for a not-very-fuel-efficient vehicle), or "My vehicle does 60 FUs" (for a more fuel-efficient thing).
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"The idea of using litres (which word many people can't even spell) per mile, or even kilometre (which most people can't even pronounce), or gallons, is still far too complicated."
Written by someone whose schooling predates the 1980s..
ALL my chilkdren learned metric at school. Just you old fuddy duddies can't adapt.. like dinosaurs...:-)
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>> "The idea of using litres (which word many people can't even spell) per mile, or
>> even kilometre (which most people can't even pronounce), or gallons, is still far too complicated."
>>
>> Written by someone whose schooling predates the 1980s..
>>
>> ALL my chilkdren learned metric at school. Just you old fuddy duddies can't adapt.. like
>> dinosaurs...:- )
What has "metric" got to do with the above?
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Oi! I'm an old fuddy duddy and I am quite comfortable with imperial and metric units. Except for Fahrenheit. I learnt at school how that scale came about and decided there and then he should have been locked up.
John
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>> I learnt at school how (Fahrenheit) came about and decided there and then he
>> should have been locked up.
What about centigrade, then? Old Celsius had zero as the boiling point of water and one hundred as its freezing point....
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"Old Celsius had zero as the boiling point of water and one hundred as its freezing point....".
Which is fair enough. It's a very useful "human" scale, covering that range which we normally experience, from a freezing day to a boiling kettle, very well. He never said there were no temperatures lower than freezing, so far as I know.
Fahrenheit on the other hand set his scale to cover the same range, freezing water to boiling water and then divided it into 180. Multiples of 12 and 60 were very popular going back to ancient times; 12 months in a year, 60 minutes in an hour, 360 defrees in a circle and so forth. He then found that if you add salt to ice it gets colder. So he fudged his scale to start at the new, lower temperature he had found, moved zero to it and we ended up with a scale with 212 degrees from 0 to boiling point. I wonder if he believed that there were no tempertaures below that?
John
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>> "Old Celsius had zero as the boiling point of water and one hundred as its
>> freezing point....".
>>
>> Which is fair enough. It's a very useful "human" scale, covering that range which we
>> normally experience, from a freezing day to a boiling kettle, very well.
Yeah, but, a very cold day would've been - say - 105C. A very hot one would've been about 70C!
>> Fahrenheit on the other hand
There are lots of interesting theories about him.
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"There are lots of interesting theories about him. "
Now that's just tantalising. Come on, spill the beans. I'll put the coffee on.
John
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Just various oddities about his scale:
www.wisegeek.com/what-is-the-history-of-the-fahrenheit-scale.htm
It's all pretty somewhat believable.
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Hmmm, thanks FT, interesting. In short, we don't know but we're very good at speculating! Just like this forum really.
John
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>> Old Celsius had zero as the boiling point of water and
>> one hundred as its freezing point....
>>
Wonder if anyone will be caught out trying to correct you!
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nah, no one reads anything I write. :-)
John
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You do whatever you like, but I will continue to think in terms of mpg. If I want mpl I just mentally multiply it by 0.22. If I want lpm I just mentally divide 1 by mpl.
Just one of the advantages of having been taught mental arithmetic in my formative years.
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Well said L. These kiddies who are umbilically attached to their phones probably want an "app" to work it out for them. Whatever one of them is :-)
John
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Never really fussed too much about mpg or whatever.
Seems to me that your car does what it does and you have to live with it until you buy another one.
When it comes to buying a car all cars the same engine size and weight tend to be about the same in terms of economy in the real world so not something I would use to differentiate between different models.
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>> When it comes to buying a car all cars the same engine size and weight
>> tend to be about the same in terms of economy in the real world so
Not so sure I agree with that, a bit of a generalization, CG! Also engines from different manufacturers are very different in the way they deliver power...
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Of course its a bit of a generalisation but its near enough in the real world. If I'm buying a car of a particular size any difference in economy between models will be fairly marginal and I don't therefore consider mpg figures when looking at what car I buy in that category.
How a car delivers power is another matter and will affect what I choose
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>> Of course its a bit of a generalisation but its near enough in the real
>> world. If I'm buying a car of a particular size any difference in economy between
>> models will be fairly marginal and I don't therefore consider mpg figures when looking at
>> what car I buy in that category.
There is some truth in that, to be sure, but the fact remains that we need a better measure of the relative fuel efficiency of cars than the current official one.
For one thing, we need it so that we can make an informed choice as to whether to buy a petrol or diesel model of our chosen car, given the relative cost price and fuel price.
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Drove up to Yorkshire last Friday to see the old folks.
Brim to brim over 450 miles I got almost 8 FPG despite heavy traffic and getting stuck in a ten mile tailback due to accidents on the M1.
Not bad for an XJ.
Kevin...
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You took the bait! :-)
John
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>FPG ?
Furlongs Per Gil
A friend in Texas was astounded that the only changes I'd need to make when I brought my Chevy back to the UK was to change the speedo from Miles Per Hour to Furlongs Per Fortnight. He thought we'd sold out and gone completely metric.
Kevin...
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MPG - time for a change ? - Tooslow
