Calculate actual combined mpg figures help please

MikeyMacbeth

Car manufacturers combined figures for mpg usage are calculated by taking an average of the cars urban mpg and the extra-urban mpg. This seems to have the implicit assumption that the car is driven under 50% urban driving conditions and 50% extra-urban conditions.

So, my question is; how would you adjust the expected combined figure if say 67% of driving is done in urban conditions and the remaining 33% extra-urban style driving? (see here for more info on how mpg is calculated if you are interested).

For example my car is a a Skoda Fabia Hatchback 5dr Elegance 1.4 16V 75bhp and has the below mfr's quoted mpg figures (source here)

Urban : 27.4mpg
Extra-urban: 47.9mpg
Combined: 37.7mpg

I'm sure some of the Excel/math whizzes on here can figure this one out - I just can't seem to get my head around it today! I guess it's a weighted average figure of sorts I'm after but my brain is obviously in complete Friday mode today:o

Toxteth_OGrady

31.9 mpg

:cool:

TOG

MikeyMacbeth

Toxteth_OGrady wrote: »

31.9 mpg

:cool:

TOG

Okaaayy...cheers for that, but how did you work it out?

BackOnTrack

I'm going to disagree and say the answer is 34.2

The calculation is:

Percentage * Urban + Percentage * Extra Urban

0.67 * 27.4 + 0.33 * 47.9

Toxteth_OGrady

For any given 100 miles you will drive 67 miles @ 27.4 mpg. You will therefore use 67/27.4 = 2.445 gallons.

You will drive the other 33 miles at 47.9 mpg. You will therefore use 33/47.9 = 0.689 gallons.

Therefore the total amount you will use to drive 100 miles is 2.445 + 0.689 = 3.134 gallons.

If it takes 3.134 gallons on average for you to travel 100 miles then the distance you can travel with 1 gallon is 100/3.134 = 31.9 mpg.

100 / ((67 / 27.4) + (33 / 47.9))

scheming_gypsy

or you can use http://www.torquecars.com/tools/uk-mpg-calculator.php

BackOnTrack

Toxteth_OGrady wrote: »

For any given 100 miles you will drive 67 miles @ 27.4 mpg. You will therefore use 67/27.4 = 2.445 gallons.

You will drive the other 33 miles at 47.9 mpg. You will therefore use 33/47.9 = 0.689 gallons.

Therefore the total amount you will use to drive 100 miles is 2.445 + 0.689 = 3.134 gallons.

If it takes 3.134 gallons on average for you to travel 100 miles then the distance you can travel with 1 gallon is 100/3.134 = 31.9 mpg.

100 / ((67 / 27.4) + (33 / 47.9))

I don't believe that is the correct method.

If you assume 50/50 urabn/extra urban driving the calculations by your method would result in:

100 / ((50 / 27.4) + (50 / 47.9)) = 34.9 mpg

Which is not the manufacturer quoted figure.

What you are doing seem to make sense so not sure why the number isn't coming out right.

MikeyMacbeth

Yep, I'm officially confused now:)

Both the above methods seem to make sense to me but I can't spot a flaw in either.

I thinks its time for some more coffee - see why I'm stuck on this now?

Actually if you just take an average of the mfr figures you actually get 37.65 so they're rounding up, but that's hardly surprising...

MikeyMacbeth

I think BackOnTrack has it!

I tried a different logic to calculate it :-

• The true combined figure has to be closer to the lower(urban) figure so...
• only 33% of the difference between the two figures will be contributed by extra-urban
• therefore the combined will be the urban plus 33% of the diffrence between the two figures

i.e. 27.4 + (0.33 x (47.9-27.4)) = 34.17

What do you think?

Toxteth_OGrady

I think there is a flaw in my initial calculation but not sure what it is yet.

Looking at it differently:

You have 1 gallon of petrol in your car and you set off at urban speed.

By the time you have used two thirds of the petrol you will have driven 27.4 / 3 * 2 = 18.27 miles.


You use up the remaining third of a gallon driving at extra urban speed.

You will drive a further 47.9 / 3 = 15.97 miles.

Therefore the total number of miles you will get out of your gallon is 18.27 + 15.97 = 34.24 mpg.

That ties in with post #4's result.

Remedial maths for me.

BackOnTrack

Think I have identified it. The numbers quoted in the OPs 1st post relate to the percentage of fuel used in different situations not the percentage of distance travelled in different situations. Quite a subtle flaw really.

So if you use half your petrol urban driving and half extra urban driving then the quoted numbers are correct.

If you do half your distance urban and half extra urban then TOG's method is correct.