Mercedes Benz OVERSTATING fuel consumption

Joe_Horner

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.

Those percentages don't affect the mileage though because, on the old cable driven ones, there's a direct (ie: "solid") drive all the way from the wheels to the odometer head. The head simply counts revolutions of the wheels and is 100% reliable at that.

Provided the tyres and final drive haven't been changed from standard, the mileage will be right even if the speedo itself (which relies on a magnetic drive internally) is out because every turn of a wheel is a known distance forwards

Joe_Horner

AdrianC wrote: »

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

That's kind of my point. The old "75 mph" figure was actually a fairly good guide to what you could expect as a minimum "average" consumption.

The newer test, while supposedly more representative, serves very little purpose except to make cars tested under it appear to be more economical than their older counterparts.

As I said above, my 1.9l diesel Pug has averaged 47mpg over the past 18 months / 20k miles or so. The Mitsubishi Colt 1.5 diesel we had as our last new car averaged just over 48mpg over 3 years / 38k miles. Yet it was supposedly far more economical than the older, bigger, and less efficient Pug - iirc official combined consumption was around 65mpg.

My driving style hasn't changed, I'm still driving the same roads, if anything I'm carrying more weight around (on my waistline and in the car) and as far as I know they haven't added extra energy content to diesel in the past few years.

Yet the test figures show the Colt as over 50% better n fuel. That's great for selling cars and pushing "green" agendas, but it's not so great for the real world economics of motorists who could well spend a lot of money to find they're getting the same RL consumption as their 20 year old barge!

AdrianC

Joe_Horner wrote: »

Provided the tyres and final drive haven't been changed from standard, the mileage will be right even if the speedo itself (which relies on a magnetic drive internally) is out because every turn of a wheel is a known distance forwards

Always assuming, of course, that those tyres are an exact known diameter - which they aren't, since the same nominal size will differ slightly between brands, as well as accounting for wear. There are various production tolerance in the actual calibration of the head.

Joe_Horner

AdrianC wrote: »

Always assuming, of course, that those tyres are an exact known diameter - which they aren't, since the same nominal size will differ slightly between brands, as well as accounting for wear. There are various production tolerance in the actual calibration of the head.

The tyre problem holds true for modern digital odometers as well.

The production tolerances for the speedo don't affect the odometer because that's a simple matter of fixed gearing. The cable drives a worm drive (effectively a single tooth gear), which drives more gears in a known ratio to turn the numbers one mile for x turns of the cable, where x depends on the final drive ratio, the speedo drive ration in the gearbox, and (nominal) rolling diameter of the tyres.

Manufacturing tolerances won't affect that ratio unless they actually use the wrong number of teeth somewhere - not impossible, but very unlikely!

sinizterguy

Joe_Horner wrote: »

The tyre problem holds true for modern digital dometers as well.

Not for the ones on a Porsche Cayenne. It corrects from sat nav data and is usually accurate to within 1mph.

Maybe some others have such features too.

almillar

Getting into the small details over the tests, speedos etc is interesting to many, including me (BTW please note how much heavier these new cars are compared to the old ones, that'll have a big effect on economy), if the OP and anyone else actually wants to save fuel, their own driving style will be the biggest factor!

Gloomendoom

sinizterguy wrote: »

Not for the ones on a Porsche Cayenne. It corrects from sat nav data and is usually accurate to within 1mph.

Maybe some others have such features too.

I've suspected that mine does for a while. The speedo is bang on compared to the sat nav.

It makes it all the more frustrating being stuck behind somebody in the outside lane of the motorway doing a steady 63 mph.

jimjames

JustinR1979 wrote: »

Is it not overstating the mpg, not consumption?

Yes. Which is the opposite of overstating the fuel consumption.

It would be interesting to see 2 drivers in same car do same route and compare economy. I bet there would be a massive difference, seeing how different people drive/accelerate/brake which all make a huge impact on the consumption.