I have often wondered why the quoted consumption figures of the 4x4 are so much worse than the 2WD, the difference being 10mpg. That is in the region of 15%! This is huge and cannot be accounted for by the increased weight alone. I accept that there will be some mechanical loss due to the more complex set up, but 15%?

Looking at the top speed (for the 2L diesel estate), there is a drop of 2 mph. Assuming most of the power is to overcome air resistance which is proportional to the square of the speed, the power drop due to the 4x4 system equates approx to 12%.

So whichever way you look at it, it is save to say that the additional mechanical loss of the 4x4 is well over 10% which is not insignificant.

Does anyone know what proportion of the loss is due to the viscous Haldex coupling to other mechanical components (rear differential for example)? If a good portion of the energy is wasted in the coupling. Does the clutch fluid get very hot and how it dissipates that heat?

Am I missing something much more obvious?

Could it be just the way the tests are done, which is on a rolling road under laboratory conditions? How would a rolling road work on a 4x4 with a viscous coupling?

Sorry, far too many questions and probably not very coherent! It is getting late after all.

Good night all

Good question - I wonder the same myself.

Considering the 2.0 litre diesel estate, 57.6mpg compared to 67.3mpg is 17% worse.

Yet the top speed of 132mph compared to 134mph is only 2mph (1.5%) different, representing a difference in power required of around 3%.  The 4x4 is 1,494mm high compared to 1,465mm for the ordinary estate (more than an inch higher) and has marginally less ground clearance (139mm compared to 140mm) presumably due to the transmission, so it will have more drag.

(As an aside: I wonder why the 1.6 litre 4x4 estate has a lower drag coefficient of 0.298 compared to the normal estate (0.304), whereas the 2.0 litre 4x4 estate has a higher drag coefficient of 0.307 compared to 0.304 for the normal estate.)

The fuel consumption test also requires acceleration so the weight will come into it as well.  The 4x4 weighs 1,380kg compared to 1,272kg; which is 108kg (8.5%) more, and this is not trivial; it is more than the weight of an extra passenger.

But even that does not match the 17% increase in fuel consumption, so (as you suggest) frictional losses in the transmission must count for quite a lot.  As you say, that must make it get quite hot!

(All the above figures are taken from the June 2013 sales brochure.  It's amazing how much information there is in there if you're bored enough or sad enough to read all the small print!)

However there is another consideration.  The fuel consumption has to be measured while driving to a very specific standardised regime, in order to provide a fair comparison between different cars.  Manufacturers will obviously tune their cars to provide the very best economy when driven exactly in this precise way, and driving in any other way (such as accelerating slighly differently, as in  most "everyday" driving) is therefore likely to produce much poorer economy.  The better the manufacturers get at "optimising" the fuel consumption in standard tests, the greater the difference is likely to be between "standard test" fuel consumption and "everyday" fuel consumption.

However, having "tuned" the engine and its electronic control unit to achieve this for the "normal" estate, it may be that they use the same engine tune for the 4x4 version.  But because the 4x4 is heavier and has more wind resistance, this particular state of tune may no longer be the optimum one for a car of slightly different weight and wind resistance, and so in the case of the 4x4 there may not be such a difference between the measured "standard" fuel consumption and the actual everyday fuel consumption.  Maybe.

It would be very interesting to learn if the actual everyday fuel consumption of readers' cars is as different between the 4x4 and the normal models as the "official" figures suggest.  Let's hope we get a lot of reports from owners.

I would be frustrated really to be 10mpg worse off all the time, just because of a feature which might only be useful once or twice a year.  

Edited 12 October, 201312 yr by Stuarted

I see 3 issues, pretty much as mentioned above:

1. The extra weight.

2. The extra height.

3. The mechanical losses in having a different gearbox, extra propshaft, haldex unit etc.

Together they add up. Also in mkII the 4x4 has a lower first gear than the fwd car.

With the official EU tests this may also make a difference. IIRC 6th gear isn't used in the EU tests either.

Sent from my Nexus 4 using Tapatalk 4

If it's anything like the mk2 I wouldn't be surprised if it had different gear ratios between the 4x4 and 2wd versions.

Good question - I wonder the same myself.

Considering the 2.0 litre diesel estate, 57.6mpg compared to 67.3mpg is 17% worse.

Yet the top speed of 132mph compared to 134mph is only 2mph (1.5%) different, representing a difference in power required of around 3%.  The 4x4 is 1,494mm high compared to 1,465mm for the ordinary estate (more than an inch higher) and has marginally less ground clearance (139mm compared to 140mm) presumably due to the transmission, so it will have more drag.

(As an aside: I wonder why the 1.6 litre 4x4 estate has a lower drag coefficient of 0.298 compared to the normal estate (0.304), whereas the 2.0 litre 4x4 estate has a higher drag coefficient of 0.307 compared to 0.304 for the normal estate.)

The fuel consumption test also requires acceleration so the weight will come into it as well.  The 4x4 weighs 1,380kg compared to 1,272kg; which is 108kg (8.5%) more, and this is not trivial; it is more than the weight of an extra passenger.

But even that does not match the 17% increase in fuel consumption, so (as you suggest) frictional losses in the transmission must count for quite a lot.  As you say, that must make it get quite hot!

(All the above figures are taken from the June 2013 sales brochure.  It's amazing how much information there is in there if you're bored enough or sad enough to read all the small print!)

However there is another consideration.  The fuel consumption has to be measured while driving to a very specific standardised regime, in order to provide a fair comparison between different cars.  Manufacturers will obviously tune their cars to provide the very best economy when driven exactly in this precise way, and driving in any other way (such as accelerating slighly differently, as in  most "everyday" driving) is therefore likely to produce much poorer economy.  The better the manufacturers get at "optimising" the fuel consumption in standard tests, the greater the difference is likely to be between "standard test" fuel consumption and "everyday" fuel consumption.

However, having "tuned" the engine and its electronic control unit to achieve this for the "normal" estate, it may be that they use the same engine tune for the 4x4 version.  But because the 4x4 is heavier and has more wind resistance, this particular state of tune may no longer be the optimum one for a car of slightly different weight and wind resistance, and so in the case of the 4x4 there may not be such a difference between the measured "standard" fuel consumption and the actual everyday fuel consumption.  Maybe.

It would be very interesting to learn if the actual everyday fuel consumption of readers' cars is as different between the 4x4 and the normal models as the "official" figures suggest.  Let's hope we get a lot of reports from owners.

I would be frustrated really to be 10mpg worse off all the time, just because of a feature which might only be useful once or twice a year.  

You are right..about 3% power loss.  Told you I was ready for bed!

 

Anyway, allow me to theorise:-

 

3% loss sounds reasonable, but why such poor percentage from consumption test.  If the rolling road used has free wheeling rollers for the back wheels, the Haldex coupling will engaged as soon as it detects a speed differential between the front and back. What happens then? As the speed matches the front, will the coupling begin to disengage again, so the system keeps hunting to and fro and as a result, a lot of energy is lost through the clutch fluid? However, if the rolling road front & rear rollers are linked to rotate at the same speed, my theory would fall down.

 

Back to the real world.  I have done 2500 miles (2L diesel  4x4), probably a bit faster than I should but definitely not thrashing it. Mixed motorway & town. I am getting about 44 mpg as an overall average.

 

Best single trip was 52mpg for a 50 mile drive on motorway at 70 to 80 mph.

The Yeti is giving computer figures of between 49 and 56 but averaging 45-47 fill to fill. Mainly open A road driving but up and down the Wolds of Yorkshire.

 

Be interesting to compare in hopefully a few (long   ) months time.

 

 

TP

I've been getting 44mpg from my mkII 4x4 too.

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I've just found this thread. Very interesting!
I've just upgraded from a 2005 Octavia 1.6 FSI to a 2015 Octavia 4x4 1.6 diesel. I was hoping for better MPG from this than I got with the old petrol version, but it's not much better. I miss my old 2003 VW Passat (2.0 TDI) that seemed to run on the sniff of fuel! I was wondering if I'd made a mistake getting a 4x4 (not by choice - there are very few affordable diesel, CAZ-compliant, pre-2017 Octavias with real handbrakes out there right now and time was running out). The salesman reckoned there wasn't much between the MPG of the 4x4 and the normal diesel, but then he would say that if he thought I was wavering!
It's still a nice, comfy car to drive (as was the old petrol one), although it still feels a little "reluctant" to move, compared to the old 2.0 Passat, which could pull away in almost any gear with no effort.

I get around 35 mpg 50:50 city:highway driving. But the Scout is even heavier than the regular 4x4 version and a bit more lifted, I believe. Also the DSG weights more than the manual as well..  

The mpgs will be lower if you do lots of town and city driving. These cars are not true 4x4 as the rear axle is on only active when pulling away or when the car senses a loss of traction.  Add in extra weight and higher drag efficiency and you have a less economic car. But the trade off is it is better in adverse weather conditions.

I get around 30-32 round town with the 1.8tsi but it can nudge 40mpg on a motorway run sticking to legal speeds which I don't think is bad for a petrol 4x4 estate. The DSG does it no favours as I am sure the constant up and down the box does not help. It seems to do better when I stick the box in manual and change myself.

 

Ironically I can get better mpg much easier with my VW, same running gear and nearly twice the power. it changes gear less and does more on the torque.