What's your MPG?

[.50]

So my reasoning still stands that a tyres rolling circumference will be affected by things like load and tyre pressure in a way that a simple mathematical calculation based on the tyres diameter/tread width/ aspect ratio will not.

If I'm going to make allowances for this in my mpg figures, then I want the allowances to he based on actual measurements and not an assumed ones.

I'm struggling to understand the point about a flat tyres rolling circumference on the van being the same as if it was pumped up.

 

[Skyliner33]

So my reasoning still stands that a tyres rolling circumference will be affected by things like load and tyre pressure in a way that a simple mathematical calculation based on the tyres diameter/tread width/ aspect ratio will not.

If I'm going to make allowances for this in my mpg figures, then I want the allowances to he based on actual measurements and not an assumed ones.

I'm struggling to understand the point about a flat tyres rolling circumference on the van being the same as if it was pumped up.

If you measured the circumference of a tyre with a piece of string and got a measurement of x mm then let, for example 5psi out of the tyre and measured the circumference again you would get the same reading. We are using the circumference as a a term loosely here as the tyres are not perfectly circular. By reducing the tyre pressure you are not removing part of the carcass of the tyre (which is pretty much made of incompressible material) therefore the distance the tyre will cover in one rotation will be the same. (providing as @Daffy said the tyre is not really flat causing deformation).

 

[Loz]

it's the distance between the centre of the axel and the road which is reduced, rolling radius which is different from the circumference of the tyre.

 

[.50]

If you measured the circumference of a tyre with a piece of string and got a measurement of x mm then let, for example 5psi out of the tyre and measured the circumference again you would get the same reading. We are using the circumference as a a term loosely here as the tyres are not perfectly circular. By reducing the tyre pressure you are not removing part of the carcass of the tyre (which is pretty much made of incompressible material) therefore the distance the tyre will cover in one rotation will be the same. (providing as @Daffy said the tyre is not really flat causing deformation).

But as soon as you put a tyre on a vehicle, the vehicles weight DOES deform the tyre, thus reducing the original radius and consequently the rolling circumference, which goes back to my reckoning that the mathematical calculation of a tyres rolling circumstance based on its diameter/width/aspect ratio, will not be the same as the actual rolling circumference of the same tyre when fitted to a functioning, loaded vehicle.

 

[Skyliner33]

But as soon as you put a tyre on a vehicle, the vehicles weight DOES deform the tyre, thus reducing the original radius and consequently the rolling circumference, which goes back to my reckoning that the mathematical calculation of a tyres rolling circumstance based on its diameter/width/aspect ratio, will not be the same as the actual rolling circumference of the same tyre when fitted to a functioning, loaded vehicle.

It deforms the side walls ad flattens the bottom of the tyre but will still leave the distance rolled by one rev the same.
By deformation it need to be flat enough so that the middle of the contact patch will deform into a sort of 'w' shape, so that the middle of the contact patch is not really in contact with the road, this will decrease the 'circumference'. If you think about a w, the distance between the two points at the bottom of the letter is less than the length of the 'upside down v' between those points. Hard to explain without diagrams. I normally have an old motorcycle tyre available and I could have take some pics

 

[ei-aprilia]

My head hurts

 

[DaveD]

And I'm still spending too much on fuel!!!

 

[.50]

I know I'm sounding like a pedant but I just can't get my head around how a reduction in a tyres radius, (either by deformation by load or low tyre pressure), doesn't then have an effect on the distance the tyre travels per revolution.

 

[Loz]

In a deflated mode, think of the deflated tyre like a tank track being fed around.
But the number of revolutions the axel will turn increases (albeit very slightly) as it gets closer to the ground.

 

[.50]

But the number of revolutions the axel will turn increases (albeit very slightly) as it gets closer to the ground.

This is exactly the point I have trying to make.

 

[Skyliner33]

This is exactly the point I have trying to make.

But it’s not correct. Think of a tank track. It doesn’t matter if it’s the traditional shape or if you take that track and put it in a perfect circle. The tank would still travel exactly the same distance for 1 complete revolution of the track.

 

[Loz]

But it’s not correct. Think of a tank track. It doesn’t matter if it’s the traditional shape or if you take that track and put it in a perfect circle. The tank would still travel exactly the same distance for 1 complete revolution of the track.

But it’s not the track distance, it’s the number of revolutions the axel, diff, gearbox and engine have to turn for that distance That consumes the fuel. And is how the TPMS works.

 

[Skyliner33]

But it’s not the track distance, it’s the number of revolutions the axel, diff, gearbox and engine have to turn for that distance That consumes the fuel. And is how the TPMS works.

But 1 complete revolution of tyre (or track) equals 1 revolution of axel/gearbox/engine therefore as a low tyre pressure does not change the 'circumference' then it wont have this effect. I thought that was how TPMS worked but after the problems I have had with it and a document that was linked eralier, Im now unconvinced its rotational differences.

 

[Loz]

No your wrong, otherwise cars would never need to have differentials. Are you also a Flat earthier?
TPMS is rotational difference as a set of compared frequency from each corner it has to reach a minimum speed for the difference to be measured.

 

[.50]

But it’s not correct. Think of a tank track. It doesn’t matter if it’s the traditional shape or if you take that track and put it in a perfect circle. The tank would still travel exactly the same distance for 1 complete revolution of the track.

A perfectly illustrated point, but I'm not sure that analogy applies in this situation as a deflated tyre will not be laid down in the same way as tank track would.

I'm now so keen to confirm my point/find out if I'm mistaken, that I am going to roll out the actually circumference of my tyre whilst off the van, and then roll out the circumference of it fitted to the van on the ground and then calculate the rolling circumference using maths and the diameter/width/aspect ratio as printed on the tyre.

I will be absolutely gobsmacked if they are all the same.

 

[DaveD]

No your wrong, otherwise cars would never need to have differentials. Are you also a Flat earthier?

The earth is obviously flat, I'm just not sure about how TPMS works .

 

[Loz]

Rolling circumference term is your issue here.

a Tyre with a larger rolling circumference also has a larger rolling radius (the distance between the centre of the axel and the road) and it’s the radius that affects gearing, mpg and TPMS.

 

[Skyliner33]

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.

Yes, at all ride heights and with different wheels and tyres. Make no difference. Even if the ride height is not changed at all.

 

[Big.mac]

Just to add to the debate here, i keep a record of all my fuel purchases and mileage brim to brim. My average consumption is 32MPG so I completely understand where @.50 is coming from. Interestingly my van originally had 16’s which I’ve changed for 19’s with 245/45’s which i think is the same setup as @.50?

 

[XZO]

Just been from Sussex to South Wales, up to North Wales and back to Sussex. Whole mixture of roads including twisty mountain lanes and worse, traffic jams and town driving let alone loads of A roads and the motorways.

Overall over 800 miles and the average was 38.56 brimmed tank to brimmed tank including the re-fuel. This on a 150 DSG Revo mapped Caravelle.

Quite happy with that.