What's your MPG?
It’s a commercial vehicle so the EU test is split between rolling road and real life road conditions, vehicles are also retested through their life to ensure that they still make the grade (up to 450,000km IIR)
Thats why no vans have been recalled to have the cheat software removed (again as far as I know)
I don’t disagree with your point about meeting the manufacturers figures though.
Even when I add back in the percentage difference between to original 16" wheels and the new 19" wheels and add that to my distance figure, it still only accounts for around 2mpg.
Just to add to the above, I didn't calculate the rolling circumstance of the 2 different tyres, I actually put a chalk mark on the sidewall with each size fitted and physical rolled it out on the floor and measured the accurate distance covered per one wheel revolution.
OK so we have established there is a difference in the rolling circumference, a chalk mark isn’t really a very accurate way to establish rolling circumference as a couple of mm inaccuracy over enough rotations to equate to a mile can be a huge difference, best use aN online calculator to be sure.
Thanks for you response, but, based on my findings, rolling out the actual circumference of a tyre using marks on the sidewall and floor is significantly more accurate than inputting the tyres diameter, tread width and aspect ratio in a mathematical calculation.
A mathematical calculation will not allow for manufacturing tolerances from different tyre brands, and will not allow for how the vehicles load will affect the rolling circumference of different tyre sizes.
I consistently find that the mathematical calculation of a tyre based on its diameter/width/aspect ratio is always signifantly MORE than the actual rolling circumference when actually rolled out and measured on the floor.
Its all fairly irrelevant anyway, as I have 2 GPS devices that put my actual speed EXACTLY the same as my vans speedo reading.
Most vehicles over record their milage anyway due to the speedo underreading and this has the effect of increasing mpg figures.
All of above is ridiculous.
I'd just like to be able to, "drive all the way back from holiday at 70mph, fully loaded", and return 40 plus mpg like so many others do.
I don’t agree with much that you have written but I‘m not going to argue with you.
You will find that the rolling circumference of a tyre will not change with load, even with a completely flat tyre the distance around the outside of the tread is the same (the rolling radius will change but that wont affect the speedo)
You are correct that speedo’s read a higher speed, this is written in the legislation, they can read up to 10% over but not lower than actual speed. This is so that the vehicle cannot be the reason you were speeding even at the extremes of tyre tread depth.
What most people dont realise is that whilst a modern electronic speedo always reads over, the odometer usually doesn’t (well it might depending on tyre tread depth but nothing like as much as the speedo) so in correcting your speedo you are likely recording less miles than you are actually doing. This will significantly impact fuel mileage.
That's not correct. The radius (distance between the ground and the wheel centre) determines effective rolling circumference and a completely flat tyre would have to rotate more times to cover a fixed distance than the same tyre inflated, therefore it would affect the speedometer.
OK, so you have me on a technicality, this is correct for a totally flat tyre as the deformation in the tyre would have this effect, I was trying to use an extreme example to make a point. However the tread band of a tyre does not change in length as it is fixed,
It's a little like pressure which does not increase with load as the internal volume of the tyre stays the same.
To (perhaps) add some confusion to this thread, I’ve just driven from the south-east to Scotland in a fully-loaded camper (and I mean fully loaded - with just me it weighs 2400kg, and I had three passengers and a load of luggage) and got the best MPG yet: it’s gone up from an average 33mpg to 42mpg.
I drove in convoy, so as you know, that means you can’t always rely on cruise-control and sometimes go either side of motorway speeds.
With the weight on board I thought it wouldn’t be as good.
SWB T28 EU5 running 90mm low on Sportline’s.
I drove in convoy, so as you know, that means you can’t always rely on cruise-control and sometimes go either side of motorway speeds.
With the weight on board I thought it wouldn’t be as good.
SWB T28 EU5 running 90mm low on Sportline’s.
What app is that?
Does the vehicles TPMS not work by detecting each wheel speed and comparing it to each other?.
As a tyre deflates, the wheel speed decreases, which the abs sensor detects and puts on the tyre warning light.
Wheel speed can only relatively decrease per the same wheel revolution if the rolling circumference has reduced.
Are you saying that if you drove with four flat tyres, or the equivalent of a tyre diameter that was the same as a standard wheel and tyre when flat, that the odometer will still record the true milage, and if so, by what means?.
I'm not arguing either, I'm just keen to learn why my understanding of this is so wrong.
Interesting, so how does TPMS work then? I thought it measured the rotational speed of the tyres, but this always puzzled me as to why having air suspension fitted makes the warning come on every journey. 
It's the radial steel belt which keeps the circumference the same (at around nominal pressures). The belt won't allow the tread to compress or stretch.
For TPMS in T6 pages 9, 20-25, especially page 22. Could say that it keeps track and measures tyre "wobble" characteristics vs. "each" speed as the "wobble" is also function of speed.
procarmanuals.com
For TPMS in T6 pages 9, 20-25, especially page 22. Could say that it keeps track and measures tyre "wobble" characteristics vs. "each" speed as the "wobble" is also function of speed.
VAG SSP 541 – Tyre Pressure Monitoring Systems 2014 – Download In PDF For Free
VAG SSP 541 – Tyre Pressure Monitoring Systems 2014 Design and function TPLI Plus TPMS Highline TPMS. Distinguishing features, Principles of tyre pressure, TPLI Plus Tyre Pressure Loss Indicator, Basic TPMS Tyre Pressure Monitoring System, Highline TPMS with trigger system
procarmanuals.com
Thanks. That still doesnt really explain why having air ride sets off the TPMS all the time.It's the radial steel belt which keeps the circumference the same (at around nominal pressures). The belt won't allow the tread to compress or stretch.
For TPMS in T6 pages 9, 20-25, especially page 22. Could say that it keeps track and measures tyre "wobble" characteristics vs. "each" speed as the "wobble" is also function of speed.
VAG SSP 541 – Tyre Pressure Monitoring Systems 2014 – Download In PDF For Free
VAG SSP 541 – Tyre Pressure Monitoring Systems 2014 Design and function TPLI Plus TPMS Highline TPMS. Distinguishing features, Principles of tyre pressure, TPLI Plus Tyre Pressure Loss Indicator, Basic TPMS Tyre Pressure Monitoring System, Highline TPMS with trigger system
You know how some questions are simple but the answers are very complex, this is one of those.
I can’t be sure without knowing a lot more about the system but I would guess that as it’s softer sprung there is more suspension movement on the road than the electronics expect to see, the fact that the rear suspension is a trailing arm design and the front isn’t means that the wheelbase changes very slightly through the range of suspension travel - in changing the wheelbase the wheel speed of both rears will differ from the fronts thus setting off the TPMS.
That’s my best guess without detailed knowledge of the system.
Not very precise, not very scientific, misleading conclusion!!!
To have any validity the same test should be conducted over, say 100 revolutions of the wheel to magnify the tiny difference in effective rolling circumference.
TPMS works by sensing, via the ABS, the cumulative effect of a tiny difference in the effective rolling circumference each time the tyre rotates, over hundreds of rotations.
If the distance between the road surface and the centre of the hub is reduced by deflation, then the effective rolling circumference is reduced, there's no way to escape a mathematical fact.
Page 18 of the TPMS manual provided by mmi confirms exactly that.
I note that at the end the words "The rolling circumference of the wheel does not change significantly when the tyre pressure is changed"
The word "significantly" is the important one, a tiny difference per revolution becomes significant when the wheel rotates so fast!
Unfortunately true. Have to admit that the air ride case is an interesting one. You probably have tried sensitivity settings of the TPMS? Is it the same at low or high ride? Have you had a chance to try with another set of tyres? A faulty tyre would be obvious one, but it probably would create other more obvious effects.
I’m going to bow out of this thread now as I normally get paid handsomely for this sort of thing
except to say that the tyre tread band does not reduce In circumference as tyre pressure or rolling radius reduces, its true that with a flat tyre it will but that’s due to deformation of the tyre and not a change in the circumference of the tread band. Driving it for any length or time in this condition would generate so much heat & change the driving characteristics so much that the OP would know he has a problem.I am pretty sure that the OP’s tyre pressures are not so far from the optimum to have an impact on rolling circumference or his TPMS.