so speedy what are u saying that, until u put the right cog in it will shift off a 3.08 diff ratio but when u swap the cog to a cog to suit 3.45 it will shift thinking its 3.45?? also interested in doing this and im just womndering if this is all i will need just a cog to suit mine being an auto aswell.

steady.
and no thats not at all what i'm saying LOL
I'm saying the auto shifts off a speed signal.

As an example, at WOT it will shift from 1st to 2nd at 80km/h.
You change your diff from a 3.08:1 ratio one to a 3.45:1 ratio diff.
Your speed is reading wrong, when it says 80, you are only doing 65.

The ECU doesn't know the speedo is wrong, so it can still shift 1st to 2nd at 80km/h because it has no way of telling the diff has been changed.
So all is good re: shifting.

Then, you get sick of your speedo being wrong, so you change the gear on the sender to the correct one for a 3.45 diff.
Now your speedo is reading right.

BUT, it hits the rev limiter before it gets to 80km/h.
The auto gives no F**k for the rev limiter, it's waiting for 80km/h to come up so it can shift.
You either get an ECU that has shift patterns for the diff ratio you are running (eg. XR, Ghia), or use a speedo corrector to correct only the speedo and make sure an unmodified speed signal is getting to the ECU.

Best way to look at it is, the speedo sender doesn't actually measure road/tyre speed, it measures tailshaft speed.
Which is obviously before the diff.
So any change you make to the diff ratio does NOT affect that signal.
It's only when you go f**k around with the right cog to fix the dash speedo that you f**k the shifts up.

That is the simplest way I can explain it, read through it a couple of times till it clicks

so you would change the cog to a 3.45 item, then use a single speedo corrector on the speedo output wire going from the dash cluster to the ECU to change the signal back.... then all should be fine?

Yep, bingo.