Worried

Oops. First sentence should read "... Great and The Good of ISO..."

I blame the slip on hearing too much of Her Nibs from Sweden on the radio news..

Hi Mike, so what's stopping anyone thinking in miles per litre. My car does about 11 miles per litre, i.e. when I buy fuel I only have the choice of buying in litres but my speedo thing only registers miles, 600 miles divided by 55 litres = about 11 miles per litre, simple.

Regards Nick.

P.S. Oh! and my car doesn't complain that it has to burn litres and drives for miles, just flashes a fuel pump icon when the equation is in danger of reaching zero miles per anything.

Edited By Nicholas Farr on 27/09/2019 00:29:06

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I’m pretty sure that fuel consumption should actually be measured in litres per km

or, of course, gallons per mile

... the inverse seems perverse.

MichaelG.

Edited By Michael Gilligan on 27/09/2019 00:32:53

And far be it from you to be perverse of course!

I suspect MPG and km/100l became established because every performance number for cars obeys the "bigger is better" principle which doesn't require explanation or understanding.

Brian

Sorry, Brian ... I think you missed my point

It was about which way up the ratio should be expressed.

There is a good clue in the expression “fuel consumption”

MichaelG.

Pedant Alert!

Only amateurs and Jeremy Clarkson take MPG seriously because it's grossly unscientific. First the heat value of fuel depends on its density rather than volume - this makes heavy fuels like diesel look excessively better than petrol and LPG. Secondly MPG is only meaningful if the engines being compared are moving the same weight over the same journey at the same speed. Even measuring MPG on the same car is dubious: Traffic Jam, cruising at 56mpg on flat empty motorway, and racing over the Alps produce different results.

The grown-ups measure performance using a basket of measures, also imperfect but less so:

The preferred measure of engine goodness is CO₂ in grams per km measured under controlled conditions. Other emissions are equally important. CO, Hydrocarbons, Nitrous Oxides and Particulates are all measured in mg/km. Ideally these should be zero.

Unlike children, grown-ups are liable to be dishonest about measurements that cost them money. They cheat, lie and obfuscate. Remember Volkswagen...

Dave

Edited By SillyOldDuffer on 27/09/2019 09:20:11

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Which are displayed convincingly [and reasonably accurately] on my dashboard display.

Analogue reading of “instantaneous” mpg

Digital reading of ‘cumulative’ mpg since last reset

Digital reading of “estimated distance available if you continue driving as you have been in the last few minutes”

MichaelG.

Ahh, yes, BUT, now we have a new boy on the block. Thanks to the auto cut off feature of most (All?) new cars to boost the MPG/km/ltr figures what price the forecourt calculators? Yet another complication, "This hybrid does 124MPG" ??????? by what parameter?

Regards Ian.

I think you missed mine as well Michael. If you express fuel consumption as fuel used/work completed lower numbers show higher efficiency, which isn't as obvious to the easily confused as it might be to engineers.

Using miles per gallon also makes the difference between cars appear greater, as whilst 50mpg is 10 more than 40 mpg 0.02 gpm is only 0.005 less than 0.025 gpm. Most people probably wouldn't realise that both of these comparisons are actually the same. Imagine trying to persuade a marketing department to claim the 2020 model "uses 4 teaspoons of petrol less for every mile" instead of "goes 10 miles further on every gallon".

Brian

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Oh well ... Let’s just agree that the whole thing is nonsense anyway

What really matters is running cost: Which should of course include some amortisation of things like the over-used starter motor mentioned by Circlip.

MichaelG.

Metric fuel consumption is measured in litres per 100km not per km, because it gives a sensible, rational number. Just like miles per gallon, rather than gallons per mile.

And to throw the spanner in the works I do miles per pound. My car does 9.45 miles per pound at today's price.

Miles per gallon was straightforward when you bought fuel by the gallon and your car/ bike measured distance in miles as do road signs. Now we have the clumsy conversion of 4.54approx. to find how many gallons we have purchased and the price of a gallon. Luckily most cars now give you a whole set of data concerning consumption in the units you prefer as well as range and travelled distance. Now I entertain myself with seeing how much I can extend the range where once it was how far round you could get the speedo and other daft speed related games.

Mike

Possibly the fairer comparison, vehicle vs vehicle is the unit used by Commercial Motor, to compare commercial vehicles. This was Ton Miles per Gallon..

It made a loaded coach weighing 14 tons, doing 20 mpg (Southdown Motor Services's Leyland Leopards did ), on a long run, look quite economical compared to a 1 ton car doing 40 mpg over the same route!

40 years ago, even a 7.5 ton ULW bus would do 10 mpg around town.

So a modern 44 ton artic, doing 6 mpg is still pretty frugal!

Howard

Re MPG, US Gallons are nearly 20% smaller than British Gallons, shock horror. All their cars are gas guzzlers and they don't have Royalty or irony either. Must be unbearable over there, poor things...

As the owner of a jeep I get the choice of US (or Queen Anne, Yanks are so behind the times) gallon/mile or litre / 100km personally the EU version takes too long to get the result

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MichaelG.

It has got to the point now that I no longer worry about the mpg, I just fill up with £20.00 a week and that does it. Even though the car has diagnostic fuel usage I dont bother looking,

here is a point, I have a class 6 engine in my VW. If I use Shell diesel within a few miles the catalyst light comes on and the book tells me I have to do at least 20 miles at 40mph to burn off the rubbish? If I use run of the mill Sainsbury Diesel, no problems. There is a price difference but it is lost by doing the 20 miles to clean the Catalyser unit.

Clive

I can get around 25 miles per Ginster on my pushbike. In contrast, a Greggs vegan sausage roll gives higher emissions and lower hill climbing ability.