Fg falcon tyres

[Fgrocks]

What's the tallest sidewall you can put on a standard g6e falcon fg,I'm just really looking for a softer ride,I have 235/40/R18s on at the moment and it's just too thuddy over bumps.

[arm79]

If your car has a 6 speed auto you can't change the tire size without upsetting the transmission.

You have to reset the tire size in the PCM to match the physical, but the size can only be one of the factory spec sizes available at the time. It won't allow you to specify a custom size tire.

[Franco Cozzo]

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

Would have to change wheels, and they're a bit pricier than the more common options too.

[JC]

Originally Posted by arm79

If your car has a 6 speed auto you can't change the tire size without upsetting the transmission.

You have to reset the tire size in the PCM to match the physical, but the size can only be one of the factory spec sizes available at the time. It won't allow you to specify a custom size tire.

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.

On an 18” rim, you can run 245/45/18 with no clearance or other issues.

[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.

[TooManyVolvos]

Originally Posted by arm79

If your car has a 6 speed auto you can't change the tire size without upsetting the transmission.

You have to reset the tire size in the PCM to match the physical, but the size can only be one of the factory spec sizes available at the time. It won't allow you to specify a custom size tire.

How does the transmission know what tyre size you're running? It can only detect revolutions as I understand it, and is therefore blind to the tyre size. Obviously the Speedo may have errors if the calibration is different than actual size. If I'm wrong I'd like to better understand.

I've searched the forum trying to get some good information on alternative tyre sizes for a stock 2013 FG Ute with 19" rims and can't seem to find anything definitive. Basically I want to alter the tyre sizes from factory. Modifications are off the table and I don't agree with rolling guards on cars that have value Anyone know if the following is an issue with stock guards and stock 19" wheels;

Front; 255/35R19
Rear 285/30R19

[78xcgxl]

I run 235 55 R17 on my FG ute for years. Car drives perfect and transmission behaving exactly the same. You can fit an even taller tyre if you run a difference offset. But with standard Falcon offset you cannot fit a taller tyre than that.

On your G6E i would run 245 45 18. That's what my brother has on his G6E and it has been fine. Ride is softer than the original 245 40 18s.

[arm79]

Originally Posted by TooManyVolvos

How does the transmission know what tyre size you're running? It can only detect revolutions as I understand it, and is therefore blind to the tyre size. Obviously the Speedo may have errors if the calibration is different than actual size. If I'm wrong I'd like to better understand.

I've searched the forum trying to get some good information on alternative tyre sizes for a stock 2013 FG Ute with 19" rims and can't seem to find anything definitive. Basically I want to alter the tyre sizes from factory. Modifications are off the table and I don't agree with rolling guards on cars that have value Anyone know if the following is an issue with stock guards and stock 19" wheels;

Front; 255/35R19
Rear 285/30R19

I couldn't tell you exactly how it knows that tyre size the car is running, I'm not that well informed. But I think I could take a pretty good educated guess as to how.

The transmission has sensors on the input and output shafts to detect shaft speeds. It also takes the input of the engine rpm. So at any one point it knows exactly what gear its in by the differences in input to output shaft speeds and any slip within the transmission and converter based on those shaft speed and engine rpm differences. You can monitor it in Forscan and watch the slip and ratios come together as it shifts.

In extension to that, it also knows at any one point of the output shaft speed what the wheel rpm should be based on the programmed diff ratio and tyre size.

So in my mind I think the transmission has an expectation of what the wheel speeds vs transmission speed SHOULD be at any one time. It uses these programmed variable to create a baseline at any speed and any variations in normal operation of wheel rpm from that baseline then determines the shift strategy and behaviour.

So if you go and change the tyre size to an extent without telling the PCM that it is changed, you are physically altering the baseline outside of what the transmission is expecting.

In the case of my car the tyres were changed to a size that was physically smaller in circumference (43mm circ and 15mm diameter), which changed the expected RPM in operation, the transmission must have always thought I slowing down when I wasn't. Or when I was slowing down or turning a corner it thought I was doing it more aggressively than I was.

And that manifested in this annoying behaviour of holding gears and aggressively and more often downshifting when it wasn't expected.

Conversely others have written here that they have changed to tyres that were larger in circumference than original and haven't suffered any ill effects. But in reality it might have had the opposite effect of my experience that this change in rpm makes the transmission less aggressive than it normally would be.

Or its possible there is an overall tolerance in tyre sizes before it starts to misbehave. And there would have to be given that overall a tyre will shrink somewhere around 12 to 15mm in diameter over its life as the tread wears down. So small changes wouldn't cause an issue.

As you said, it all has to do with tyre rpm... And what Theo from ZF told me in that email. By changing tyres you are changing what the expected rpms should be which causes issue with the logic that is solely trying to detect variations in tyre speeds.

That's my theory... If that makes sense.