[arm79]

Originally Posted by Franco Cozzo

I think the FGII EcoLPI XR6 came with 235/50R17 - can that be picked in the PCM options on the reflash?

There is a list of tyre sizes printed in the respective WSM's, but I can't imagine its a complete list. It probably changes and it added to as different options come on board.

The problem of only being able to program factory fitment options seems to stem from the tyre size being converted into a calculated circumference and tyre RPM figure that the PCM uses. I think tuning software get around that by being able to program the circumference and RPM figure directly.

Originally Posted by JC

Not too sure that’s right. I run 245/40/19 and 275/35/19 - one size up from the standard 245/35/19. Have had no transmission issues, no weird gear changes, nothing.

No, that is correct.

The transmission constantly and fastidiously monitors the rotation speeds of the front wheels to determine how it operates at any one time and the physical wheel rotation speed needs to match the calculated size programmed into the PCM for the trans to perform properly.

From the ZF Australia god himself, I paraphrase some parts of his reply to me from a while back:

First steps for me would be to make sure as you mentioned that the tyres and tyre pressures are correct. This will influence the adaptive shift strategy.

As the tyre rpms is critical for the shift strategy and cornering mode.

My problem was that the previous owner put on tyres that had a smaller rolling circumference that what was programmed, smaller by about 15mm. Ths caused the transmission to aggressively downshift constantly.

As soon as I took my foot of the accelerator it would downshift. Turning corners it would shift down an extra gear. It would shift down an extra gear in downhill mode. It would hold gears before upshifting in a straight line, like it would hold 4th forever before shifting into 5th at 60km/h... Same with moving into 6th at 100km/h. It basically thought it was constantly in cornering mode and was an extremely frustrating car to drive.

In your car your tyres work out to be a larger rolling circumference than the original, but about 15mm. It might simply be that because is the opposite case to me that its delaying downshifting and isn't operating in as sporty a manner as it should. Maybe its just something you haven't noticed or are used to.

I suppose its possible that if you find a tyre close enough in circumference size to the programmed originals you won't ever notice a problem.

If you're ever unfortunate enough to blow a front tire and have to put on a different size spare you will see. You'll think the transmission has gone mental. In my car a spare on the front will see it hold 1st to 4th to nearly at the redline. It won't shift past 3rd under 60km/h and will be lucky to shift into 5th at 100km/h. If I want it to shift up I have to manually shift.

This video is always a good demonstration of how the transmission behaves with when it sees different rolling sizes between the fronts.