Finally upgraded!!

[drover]

Originally Posted by GCDingo

I run 275/65/18 Wilpeak AT3W and have a 255/70/18 dueler HT that just fits in the spare well. Using drovers tyre calc page a 275 vs 255 is 1mm in diameter and circumference and revs per mile is the same. I hadn't changed anything else when that happened so 1mm and it picked it up is crazy.

But if you hadn't changed the wheel size in the BCM it will be seeing the standard wheel size and the Dueler's actual size may be a bit bigger but more likely the dynamics of a narrow tyre means it has less grip so in fact may turn faster with a bit slip than the other wheels which will have more drag... thats why those skinny spares have a limited speed and distance rating...

Width, height, circumference, tread pattern and tyre compound/build all play a part when one is different the whole dynamics change and not just the wheel the suspension will operate slightly out of synch to the rest as well...

[OLMATE]

Originally Posted by GCDingo

I 275 vs 255 is 1mm in diameter and circumference and revs per mile is the same. I hadn't changed anything else when that happened so 1mm and it picked it up is crazy.

If that was the case why doesn't it effect those who run 20's when the vehicle had 18's as STD fitment I personally change between 18's & 20's
and the GPS speed is the same, when comparing it to the digital speedometer, vehicle always reads 1.5kph faster than gps

[FARWEST]

OL MATE that has to be the $64 question....seems miniscule differences make major issues if left unattended...but....it would seem that only a minor time period isnt going to produce an immediate breakdown....however...extended periods of differing tyres will cost you a transfer case....if you watch that Vid I posted you will get the 'guts' of it....computer/sensory override of clutches....wasnt an issue when we stopped, depressed the clutch, selected Neutral, picked which drive mode we wanted WITH A LEVER, selected our desired gear and away we went....only blew a transfer case if 'the nut behind the wheel' lost a strong grip on reality....or didnt have one to start with

[LeCheese]

Originally Posted by OLMATE

If that was the case why doesn't it effect those who run 20's when the vehicle had 18's as STD fitment I personally change between 18's & 20's
and the GPS speed is the same, when comparing it to the digital speedometer, vehicle always reads 1.5kph faster than gps

Rolling diameter / circumference of the tyre is the same between stock 18s and stock 20s. Also if you're changing all four, then there's no discrepancy between the wheels on the vehicle, which is the supposed issue.

In saying that, not calling BS on your observations GCD, but I'm really struggling to see how 1mm discrepancy will make a difference. That would surely be within tolerances to allow for uneven wear - even differing tyre pressures. I've seen people report running the same 275 & 255 spare combo with no issues previously too... coincidence maybe?

On Drover's point on adjusting the tyre size in the vehicle's brains - I've run 275/55/20 and 275/65/18 for over 10 years across two GCs without updating the tyre size in the module and haven't had any issues whatsoever. The speedo reading (and no doubt odo) is of course about 5% lower than on stock tyres due to the increase in circumference, but now it reads pretty much spot on to the GPS derived speed, which I prefer anyway.

[drover]

Changing the settings is more about improving data information for things like traction control etc as well as other things like speedo...........

I'm not saying not changing it makes things worse but it is a possible cause......
. Same tyres, same wear should mean the data sent by the sensors is similar so its all happy, the algorithum is more likely that revs per ms of all wheels should only vary by "X" amount, not neccessarily a set amount for a certain size, that would be why larger wheels all identical doesn't cause wobblies, the data is all in synch...... so traction control doesn't have a problem at all.... If the system is happy then its all good, you don;t want traction control putting the brake on one wheel because it thinks its slipping as you drive straight down the road ..................

GPS accuracy is reliant on the number of satellites picked up 5 or more is good but under that accuracy can be dodgy

PS: looked at the tread on the AT3 and the Dueller HT, vast difference one being reasonably aggresive ATR and the other highway pattern that could make a difference especially on the rear.......... well I think it might.

[GCDingo]

Originally Posted by LeCheese

Rolling diameter / circumference of the tyre is the same between stock 18s and stock 20s. Also if you're changing all four, then there's no discrepancy between the wheels on the vehicle, which is the supposed issue.

In saying that, not calling BS on your observations GCD, but I'm really struggling to see how 1mm discrepancy will make a difference. That would surely be within tolerances to allow for uneven wear - even differing tyre pressures. I've seen people report running the same 275 & 255 spare combo with no issues previously too... coincidence maybe?
.

I am 100% with you LeCheese, that's what I can't understand. Nothing else changed that day on the car, other than me changing to the spare. That was the only reason I could see for it to be making the kind of ABS click noise that it did. Some day soon when I can be bothered, I might chuck the spare back on and take for a drive again and see if it comes back.