Mpg is particularly misleading when you are looking at improvements or deterioration in fuel consumption figures.
Which looks like the best improvement?
56 mpg to 62 mpg
or
14mpg to 15.4 mpg

A lot of people would get excited about 56 mpg improved to 62 mpg about 6 mpg better or a 10% improvement. It would save less than 100 litres in a normal year (assuming about 20,000km/year).
or
At first glance few people would think 14 mpg improved to 15.4 mpg was worth the trouble. After all it is only about 1.4 mpg better. It is also a 10% improvement. But it would save nearly 400 litres in a year.

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1973 Series III LWB 1983 - 2006
1998 300 Tdi Defender Trayback 2006 -
- often fitted with a Trayon slide-on camper.

A 100% improvement is a ratio improvement therefore it has no bearing on the actual outcome relative to 100% improvement on a different vehicle. A percentage is proportional only to the base range that it is compared to.

L/km is simply an inversion of km/L and by fixing the denominator at 100km you scale the numerator. At the end of the day it doesn't matter because either method gives you an indication of how fuel efficient the vehicle is the actual difference is that when talking in mpg (or km/L) the larger the numerator the more efficient the fuel consumption where ase in L/km the inverse applies. If we were to talk in terms of percentage increase then this would be only relevant if we compared this weeks fuel consumption with last weeks.

I still think kJ/km would be even better (especially for those with a V8)

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08 Defender 110 TDCi
99 Defender 110 300Tdi
08 Kimberley Kamper Platinum
iMac
MacBookPro
flickr

Past: 03 D2a Td5, 97 D1 Tdi

If you mean you measure the energy input required to cover a set distance, then I would agree with you because it is using the same logic of measuring fuel (or energy) used over a set distance. After all it is a fuel consumption figure, so it should be stated as an amount of fuel (or energy) consumed over a set distance.

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1973 Series III LWB 1983 - 2006
1998 300 Tdi Defender Trayback 2006 -
- often fitted with a Trayon slide-on camper.

My point exactly. It's a bit like some obscure sport claiming they are the fastest growing sport in Australia. Cross-country tiddlywinks could probably quite correctly claim they were the fastest growing sport because last month there was one person competing and now there are two, a 100% growth.

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1973 Series III LWB 1983 - 2006
1998 300 Tdi Defender Trayback 2006 -
- often fitted with a Trayon slide-on camper.

Hi all, you could use the following fuel economy calculator and get the figures in MPG, Litres per 100 or kms per litre.

http://www.dropbears.com/u/utilities...fficiency.html

Greg

I was just trying to make the maths simple by using 100%.
By the way a 100% improvement on 28 mpg is 56 mpg, (not 14 mpg as you state)
I would have though that it was more important to know the actual change in consumption rather than the percentage. After all as you point out yourself, it depends on what it is a percentage of. That was one of the points I was trying to make.

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1973 Series III LWB 1983 - 2006
1998 300 Tdi Defender Trayback 2006 -
- often fitted with a Trayon slide-on camper.

Yes I see your point - and sorry yes 100% on 28 is 56 - blonde moment

But a gpm improvement is exactly the same as a l/100km

Oh bugger it I give up

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'96 Disco 1 V8 - Sadly Rolled
2002 Disco II V8 - happy as larry

Whether the better way of expressing it is litre/km or km/litre depends on whether you are trying to work out how much fuel you need to get from A to B or trying to work out whether you can get from A to B on one tank.
With the ready availability of pocket calculators, the practical difference is zero - the only advantage of either method is that most people can multiply in their head easier than they can divide.
(Since you are buying fuel in litres, if you have a speedo in km, which is everyone except early Series owners, miles don't come into it, and /100km is a bit silly as well).

As I said above - there is no real advantage to either, it is simply that some people are more familiar with what they grew up with. For example, I grew up with the idea that a reasonable fuel consumption figure is 30mpg (9.42l/100km), this being the sort of figure you got from a Holden (48/215) or my father's Swift, and consequently I use it as a yardstick to this day, so I have to convert either way to compare. But I don't think that either method is inherently better, although I do think the 100 is silly - it is there only so you deal with whole numbers, and the simpler way would be to just use km/l. The argument over whether a smaller number is better or a higher number is better simply depends on whether you are playing golf or cricket - and seeing that we were used to having a higher number as better from the start of motoring, and the rest of the world still works that way, it is simply a change to be different foisted on us by the metric conversion board.

John

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JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol

That's right. I would have thought that the former situation is more common in the real world.
Exactly! Working out L/100k is a simple multiplication. eg if you have a 15,000 km trip planned and you use 11 L/100km just multiply the two figures and knock a couple of 0s if the end. You have to budget for 1650 litres.
That is actually one of the reasons I found it so easy to start thinking in litres/100km. Perhaps our family was bigger and we had more kids loaded into Dad's FJ Holden, but we generally got about 28mpg. I had always seen that as some sort of benchmark. Since that was about 10 L/100k, it seemed to offer such an easy way of calculating fuel consumption. I didn't even really have to do any multiplication, since multiplying by 10 just involves adding a 0. Perhaps if my parents had owned a thirsty Yank tank that used something like 18mpg and I had become accustomed to that figure, I would not have found the transistion so easy. I suspect that part of the reason I changed was because when I used L/100km, there was really no maths involved. In fact because of the need to divide by 100. All I had to do was remove a 0. So 670 km would need 67 litres. What could be easier?

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1973 Series III LWB 1983 - 2006
1998 300 Tdi Defender Trayback 2006 -
- often fitted with a Trayon slide-on camper.

Provided your claim that the former case is more common - but my belief is that it is equally common to put your example as "My tank holds 80l - will this take me more than 670km?" Using your figures the answer is it will take me 800km so I have a margin.

You can make the calculation either way - but your main concern is "Am I going to run out of fuel?", which you can answer by either calculating the fuel required for your trip and comparing it to how much you have, or calculate how far you can go on the fuel you have and compare this to the distance you have to go.

And for your figures, (670km, 10l/100km or 10km/l) the calculation is equally easy - in fact, by simply dropping the zero you have actually divided by ten (which is the calculation for 10km/l), where if you had actually used 10l/100km you should have divided 670 by 100 to get 6.7 and then multiplied it by ten to get 67, two steps rather than one. For a figure other than ten the calculation is a bit more complicated, but most people will have to use a calculator anyway if either the km/l or l/100km has anything after the decimal (as is usually the case)!

John

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JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol