Thread: MPG vs L/100k

But some of us grew up before metrification

L/100km is certainly the best for comparing fuel consumption rates but in fact it is easy to use either in calculating consumption for given distance.

Now if we were to also factor in the calorific content of the fuel...

G'day Folks

It's simple really, 10 Gallons @ 18 miles to the Imperial Gallon gives a range of 180 miles or 175 with safety

And I think in Lbs per Sq inch and Ft Lbs and it is only 653 miles from here to Sydney and speed is 60 mph is a mile a minute

There is not really any logical preference to either system - some of us grew up with mpg so tend to think in it, but either system works perfectly well, although if your speedo reads in km the metric system is probably easiest. The only logical advantage to mpg over litres/100km is that it is in the form unit/unit rather than unit/100units. As far as I know Australia is the only place this particular unit is used, elsewhere it is km/litre (except for the UK and the US who still officially use imperial units, but the Americans use a wine gallon not an imperial gallon - which maybe says something about the US culture in 1776).

Just to confuse matters my 2a has the dipstick in the right tank in gallons and the dipstick in the left tank in litres!

John

Originally Posted by dm_td5

But some of us grew up before metrification

Quite a few of us actually, I still work an abacus

Originally Posted by vnx205

In case anyone is still working in mpg instead of Litres/100km, there really are good reasons to change.
One important reason is that mpg makes it hard to work out the savings that have been achieved with improvements in fuel economy.
Who has saved the most money?
Driver A improves her fuel consumption from 40 mpg to 60 mpg. (50% better)
Driver B improves her fuel consumption from 20 mpg to 25 mpg. (25% better)
In fact driver B saves more money than driver A. For every 100km they travel, driver A saves 2.4 litres while driver B saves 2.8 litres over the same distance.
Another example:
Both drivers improve their fuel economy by 5 mpg (from 40 to 45 and from 20 to 25). Driver B is saving 2.8 litres for every 100 km; driver A is saving just .8 litres over the same distance.
Another reason is that it is a bit illogical to talk about fuel consumption in mpg. Almost everyone finds themselves most of the time in the position of having a set distance they need to cover rather than having a set amount of fuel to work with. That is you need to know how much fuel you will use to cover that distance. Whether that is the distance you expect to cover to get from point A to point B, the distance you expect to do in a year or the distance you need to cover on that trip around Oz. If you know how many litres you consume for every 100km, it is dead easy to work out how much fuel you need.
It is fairly rare to find yourself in the position of having a set amount of fuel and you just drive as far as you can on that fuel, but that is what mpg lends itself to.
The same problems, distortions, lack of logic, difficulties and inconvenience apply to km/litre.
Litres/100 km really is the only sensible way to work out fuel consumption.

I really do not know how that logic works, whether it's 15 l/100km or 20 mpg, and you save 20% - it's 20% in either language, makes absolutely no difference

.............but everyone to their own

Originally Posted by vnx205

In case anyone is still working in mpg instead of Litres/100km, there really are good reasons to change.
One important reason is that mpg makes it hard to work out the savings that have been achieved with improvements in fuel economy.
Who has saved the most money?
Driver A improves her fuel consumption from 40 mpg to 60 mpg. (50% better)
Driver B improves her fuel consumption from 20 mpg to 25 mpg. (25% better)
In fact driver B saves more money than driver A. For every 100km they travel, driver A saves 2.4 litres while driver B saves 2.8 litres over the same distance.
.
Litres/100 km really is the only sensible way to work out fuel consumption.

40 mpg = 7.0634 l/100
60 mpg = 4.7089 l/100

7.0634- 4.7089 = 2.3545 or 50.001061819108496676506190405403%



20 mpg = 14.1268 l/100
25 mpg = 11.3014 l/100

14.1258 - 11.3013 = 2.8245 or 24.99269995487244830240768761116%

Sorry I could only calculate this to 31 decimal places

But whatever way you look at it the savings in % terms are, and always will be the same irrespective of whether you use metric or imperial - that is one of the beauties about %'s

Just my opinion - and I am quite surprised knowing your age - that you you do not think imperial

Originally Posted by ladas

Quite a few of us actually, I still work an abacus

Didn't you move to the slide rule with the rest of us?

It really has nothing to do with imperial or metric. The point is do you really want to know how far you can go on a set amount of fuel or do you want to know how much fuel you need to cover a set distance. Km/litre is just as bad as mpg.
The problem with dealing with percentages is that a high figure for mpg really means a small amount of fuel used, so the same percentage change would mean a smaller saving in fuel than a low mpg figure with the same percentage change.
A 100% improvement on 28mpg will save you about 5 litres every 100 km or about 1000 litres a year.
A 100% improvement on 56 mpg will save about 2.5 litres every 100 km or about 500 litres a year. They are both 100% improvements, but one saves twice as much as the other.