dash recalibration

[risingskull]

hey everyone i need a lil help.

my speedo is out by about 15 kms
i need to figure out what my ppk is so i can recalibrate my dash cluster.

i went from 18inch wheels to 20 inch and i went from (3.46 diff back up diff) to a 3.9 diff.
i have a vx ss 6 speed manual,

any help would be appreciated
thank you in advance

 

[krusing]

There is a supple equation to figure that out.
What exactly is the size of the tyres that you have fitted ?

 

[risingskull]

There is a supple equation to figure that out.
What exactly is the size of the tyres that you have fitted ?

235/35/20 rear and 225/30/20 fronts