Setting speedo for different diff ratio

[ToyTA22]

I am changing my TA22 diff from a standard 4.1:1 to a 4.3:1 lsd with T50 gearbox. I don't know how to calculate speedo gears required in a TA22. Other manufacturers use 1000 cable revs per mile. If I knew where to find speedo calibration number I can calculate from there. Only number I can see on back of speedo is 057100 - 6050
Also I am changing to a GT dash assembly so have access to back of speedo at the moment.

Any help would be appreciated.

[GT1978]

On my RA28 or maybe it's my RA40? I remember seeing it printed on the face down the bottom.

Glen

[dumbass]

change the diff then see how far speedo is out
if its 10% to fast then on a 20 tooth gear u need to put a 22 tooth gear in the speedo drive
I think that's right

[bmxer54]

I'm not sure if i have this right or not but i'm hoping that the speedometer is calibrated to the wheel size and then the speedo drive is calibrated to the diff ratio
I'm hoping it all works this way as I have thrown a hilux diff under my rt142 and my big problem is that it was built so long ago that I dont know the ratio.
I have done some road testing (comparing gps speed to dash) and come to the conclusion that the speedo is 25% out. considering the stock diff was a 3.89 bw and it is now 25% out this brings me to the conclusion that my diff is a 4.86. Was 4.86 a hilux ratio? and is the first line of this post correct meaning all i need to do is find the speedo drive from something with a G142 as the diff code and a w5x box? Or is it as simple as dumbass has said and if say the current speedo drive is say 20 teeth the just get one with 25 teeth?
Sorry for the massive post but there is alot rattling around in my head

Cheers,

Peter