Thread: Calculating Average Fuel Consumption

Originally Posted by Ferret

The series of figures given (12l/100, 13l/100, 10l/100 and 15l/100) where not a series of 'numbers' they are a series of 'scalers'. That is, they have units associated with them (ie l/100k) and those units must be must be taken into account when the doing any maths on them, eg when determining the average of the series.

If you were asked add 3 + 4 together the correct answers is 7 but if you were asked to add 3 mm + 4 m together the correct answer not 7 but 4.03m

Since the units are specified in the original question then there is no doubt over what was required.

Oh I think given this thread has stretched to some five pages means there was plenty of doubt over what was required by the OP.

He has given weighting of the distances (abovew somewhere) as the reason for not going down the path of 12.5l/100km but you have indicated another reason but it is not clear how or why it should be applied, please elaborate...

Originally Posted by windsock

Yep, certainly on the same page, literally, but certainly not figuratively.


Got me laughing though. There is something surreal about disagreements on the internet...

I believe that another figure of speech with the same meaning is "singing from the same hymn sheet."

This cartoon is the best laugh I have had for a while.

http://imgs.xkcd.com/comics/duty_calls.png

It was posted by akelly when he and I were involved in a similar "exchange of opinions".

Originally Posted by vnx205

I believe that another figure of speech with the same meaning is "singing from the same hymn sheet."

This cartoon is the best laugh I have had for a while.

http://imgs.xkcd.com/comics/duty_calls.png

It was posted by akelly when he and I were involved in a similar "exchange of opinions".

yes, indeed, my favourite gif to depict such things is here. I tried to post this as a gif on this site but I cannot. Is there a way to post animated .gif on here?

Originally Posted by windsock

He has given weighting of the distances (abovew somewhere) as the reason for not going down the path of 12.5l/100km but you have indicated another reason but it is not clear how or why it should be applied, please elaborate...

Put it this way, if the question had been posed as

A person travels the following distances and records for each distance the following respective fuel consumption figures.

300km, 200km, 400km, 100km and 12gallons/100 miles, 13 pints/100 yards, 10 litres/100 kilometers and 15 buckets/100 feet.

Would you still believe the average fuel consumption would be the sum of (12 + 13 + 10 + 15) / 4 = 12.5?

And if you did believe this what would the answer represent 12. 5 mile / gallon, 12.5 l/100k or maybe something else.

I would say you would probably not do this because you would recognise you just can't ignore the units when doing the maths, ie they are not just numbers which you average.

Originally Posted by Ferret

Put it this way, if the question had been posed as

A person travels the following distances and records for each distance the following respective fuel consumption figures.

300km, 200km, 400km, 100km and 12gallons/100 miles, 13 pints/100 yards, 10 litres/100 kilometers and 15 buckets/100 feet.

Would you still believe the average fuel consumption would be the sum of (12 + 13 + 10 + 15) / 4 = 12.5?

And if you did believe this what would the answer represent 12. 5 mile / gallon, 12.5 l/100k or maybe something else.

I would say you would probably not do this because you would recognise you just can't ignore the units when doing the maths, ie they are not just numbers which you average.

But the example we were given does not have different units.

I think the best explanation of why just doing an average of the two numbers given is not correct so far has been the one relating to 1 person weighing 100kg vs 99 weighing 200kg, the average weight of the group is not 150kg but ((100 x 1) + (99 x 200))/100 = 199 kg, it is a weighted average issue not one of mixed units.

Anyhow, the factual answer is 11.7, cause the rest of it means ****, as the average fuel used to do the kilometres, is 11.7L/100kms

Originally Posted by slug_burner

But the example we were given does not have different units.

Maybe not, but you are manipulating the units just as you are manipulating the numbers when you do the maths. The fact that the units are all the same means you just don't see it as clearly.

For example, for the first trip section 300 km @ 12 l/100km;

3 * 100 km * 12 litres / 100 km = 36 litres.

The 'km' units cancel out and you are left with units of litres, ie you are converting a measurement with units of litres/100k into a measurement with units of litres

But I think I am boring people