Mercedes Benz OVERSTATING fuel consumption

BeenThroughItAll

sh0597 wrote: »

The fairest way to tax CO2 production is to increase fuel duty. The present system is imperfect for a few obvious reasons.

We're already paying proportionately more tax on our road fuel in the UK than in many other countries, so surely we are already paying a sliding scale of taxation based on consumption?


I know I pay more tax via fuel duty than my colleagues in their 60+MPG euro-boredom-boxes. I pay more via VED and income tax than a lot of them too, but ultimately they're irrelevant - my tax burden is directly related to the amount of fuel I use, and if I choose to run a V8 Range Rover then I'll pay a lot more in taxes.

colino

As has been said, the figures are only there as a comparison tool, they certainly can't be relied on for real people in the real world. If I borrow my Wifes car even for a short time, not long distance on decent roads, I'll average 29-30mpg according to the cars little computer. She, driving in the same area and conditions, will rejoice if she averages 23mpg.

sh0597

There's a few things to consider.


Heavier cars will be more affected by gradient changes. Driving a heavy car up and down hills will burn more fuel.


"Stop/Start" mechanisms won't save you fuel if you are rarely stationary.


Drag coefficient will determine how efficient the car is at high speed. A lot of SUV's have poor drag coefficients.


Some low power engines are operating well beyond peak efficiency at motorway speeds, this will hurt mpg.


Then there are other small factors. Tyre quality, condition and pressure. Quality and viscosity of engine oil. Quality of fuel. Ambient temperature. Friction of the road surface. Vehicle load including weight of fuel.

almillar

Merc are the worst offender, E220cdi - they state 61.4mpg and what you get is mid 40's at best.

No, what YOU get is mid 40s at best.
I'd get different.
That other guy would get different.
Her over there, would get different.
The test is standardised. An run that simulates urban driving, and a run that simulates 'extra urban' (out of town) driving. All cars do the same route. The 'combined' figure puts the two together.
You decide what MPG YOU will get, by finding out where you compare to the official figure for your CURRENT car. If you're getting the urban figure in your current car, there's no point looking at the extra urban for your next car - that's not your driving pattern!
The range of figures is a scale. Find out where you are on that scale now, in your current car, and even previous cars. As long as your drives haven't changed, you'll be in roughly the same place in your new car.

Of course now that the test has been around a few years, the manufacturers can play around with gear ratios etc to get better results in a test, but it's still a great, standardised way of comparing cars.
Until you get into hyrbids/electrics, which is a minefield so far.

Easy test:
20+ year old Citroen AX diesel - 60+ mpg
New Fiesta 1.6TDCi - 60+ mpg

The Citroen AX will output closer to double the CO2 emissions of the fiesta.

The Fiesta does have a cleaner engine through, for example, higher pressure injectors. But it also does the very low tech trick of shoving a lot of the dirty stuff into a box in the exhaust. Genius! (Granted, not the CO2)

Joe_Horner

almillar wrote: »

No, what YOU get is mid 40s at best.
I'd get different.
That other guy would get different.
Her over there, would get different.
The test is standardised. An run that simulates urban driving, and a run that simulates 'extra urban' (out of town) driving. All cars do the same route. The 'combined' figure puts the two together.

Spot on, although a curious thing I've found is that in our usual old cars (currently '94 BMW 525 td, Pug 405 GLXD estate and '74 Daf 66) I can consistently beat the official figures by around 15 - 20% .

As an example, the Pug was given an overall consumption of 39mpg by the old test, I've averaged 46mph over the 18 months I've had it, and a good deal of that's been round country lanes.

That's been true for all the cars I've owned or driven except the few newer ones that were tested under the current regime, where I've struggled to reach the quoted "combined" figure on any of them. But that's not a fault of any particular maker, rather of the test regime itself.

AdrianC

Joe_Horner wrote: »

Spot on, although a curious thing I've found is that in our usual old cars (currently '94 BMW 525 td, Pug 405 GLXD estate and '74 Daf 66) I can consistently beat the official figures by around 15 - 20% .

Those would have been very differently calculated figures, though - constant 56/75mph and "urban cycle", rather than the current fairly complicated "test route".

redux

Joe_Horner wrote: »

Spot on, although a curious thing I've found is that in our usual old cars (currently '94 BMW 525 td, Pug 405 GLXD estate and '74 Daf 66) I can consistently beat the official figures by around 15 - 20% .

As an example, the Pug was given an overall consumption of 39mpg by the old test, I've averaged 46mph over the 18 months I've had it, and a good deal of that's been round country lanes.

That's been true for all the cars I've owned or driven except the few newer ones that were tested under the current regime, where I've struggled to reach the quoted "combined" figure on any of them. But that's not a fault of any particular maker, rather of the test regime itself.

There seem to be comments in places that official figures overstate what typical drivers will achieve by 20 to 30% nowadays, whereas it used to be 5 to 8%.

And maybe it is to do with manipulation by the manufacturers, with rumours of unhomologated modifications such as removal of alternator and air conditioning drives, extra fairings and sealing, and overinflated slick tyres.

For me, it's just unbelievable that the same marque of car can have improved from 56 or 62 to over 80 mpg in just a few years.

Mind you, VW and Audi did it with the Lupo and A2 3l TDI, but those are a special case with more extensive changes.

sh0597

I doubt those things are true. Sounds like pub rumours.

JustinR1979

Joe_Horner wrote: »

Spot on, although a curious thing I've found is that in our usual old cars (currently '94 BMW 525 td, Pug 405 GLXD estate and '74 Daf 66) I can consistently beat the official figures by around 15 - 20% .

I reckon speedo makes a difference between old and new cars.
Old cable driven speedos can over read by 10%, whereas newer abs sensor driven speedos can be just 1 or 2 mph over.
So there's a few percent difference to go towards the mpg showing better on older cars.

FiremanDave

JustinR1979 wrote: »

I reckon speedo makes a difference between old and new cars.
Old cable driven speedos can over read by 10%, whereas newer abs sensor driven speedos can be just 1 or 2 mph over.
So there's a few percent difference to go towards the mpg showing better on older cars.

I'm sure at one point German cars were 16%.