wheel size - why does the car need to be told?

[jagshield]

I'm thinking about getting some s/h wheels for my 2016 XF. It's currently on 19", but I prefer the ride of smaller sizes. Accepted information (as in this month's Jag Enthusiast magazine) says that if you change wheels, you need the dealer to 'tell' the car that you have done this. But why? The rolling diameter is pretty much the same for every wheel size on any car, and so the effect on the speedo reading is negligible. And as my suspension is a passive one, I can't see any effect there, either. Has anyone got a dealer to do this (cost?), or did you just drive on happily?

[J44EAG]

I recently also saw that in the magazine and immediately scratched my head. I thought little more of the matter until an X350 owning member contacted me in my role as X350 Co-ordinator. He too thought the statement strange.

I then contacted our Editor, Nigel, and voiced our concerns. Nigel was seeing Dave Marks that coming weekend and promised to speak to Dave about the issue. I`ve not yet heard anything back by return so will make contact again next week.

Whether a car has small or large rims fitted, the rolling circumference of the TYRE is the important thing. In all cases seen in my fifteen year dealings with Jaguar confirms that what ever size of rim you have, the tyre circumference stays the same but obviously type height ie %value alters.

Like Jagshield, I`m mystified at the published statement. Seems like a red herring to me.

Mike K

[jagshield]

Hi Mike,
Thanks for the confirmation of your thoughts (and not commenting on my use of 'rolling diameter', when of course it should have been 'rolling circumference'). It would be great if we can have some further details about this.
It's not just in the magazine that I heard this comment about 'telling' the car, though. I did once hear it from a dealer, when I was thinking about buying a particular car but wanted a different set of wheels. He couldn't say why, just that it 'had' to be done.
Could it be relevant, though, for cars with an active suspension? Here the controlling computer is assuming a certain compliance in the tyre, which will be dependent on wheel size.
Now onto the other thing in this potential change. I presume if I buy wheels without TPMS sensors, then I would be advised to buy new ones as those from the original wheels will risk being damaged in removal/refit. However, as I check tyre pressures myself on a regular basis, I don't really need TPMS. But will the car continue to throw up error messages if I don't have sensors fitted?

[rosemary]

Hello Folks
Not sure if I've got the question right but the following may be of interest. The rolling distance of the wheel at the correct tyre pressure is used in the ECU as one of the parameters that contribute to the annunciation of a tyre pressure alarm along with the ABS sensor which counts the revolutions of the wheel, the TPMS is a further parameter in the alarm process. If any of these parameters are out of the set range at development of the software an alarm is triggered.
The speed and distance measurements are set by the rolling distance of the wheels within the ECU and tachometer at development in addition, at the correct tyre pressures.
Finally in respect of TPMS this is designed as an instantaneous indication of tyre pressure loss to dangerous levels, the counting of wheel revolutions is a much slower process to indicate a loss of rolling distance, slower that is in electronic terms, for clarity.
Dave

[jagshield]

Hi Dave,

Thanks for this. You could be on to something that I had not considered here, as my car does have a TPMS system. It seems an overly complicated means of checking for low tyre pressures, when a more simple pressure monitoring would have sufficed.
It doesn't explain why this was a requirement on pre-TPMS cars, though.