How to transfer ratio to new tranny ?

[SpeedAssault]

I read around but there was not anything of final output when using tranny and spur/pinion.

I am changing my emaxx tranny to a direct drive Losi 8 E (losa3562)
I want to keep roughly the same output ratio with my old Emaxx tranny locked in 2nd (31/18 = 1.72:1 With 58 Spur and a 21 pinion And find what spur and pinion to use on new Losi center diff . stock part is a 45t spur so
What pinion do I use ?

How do you calculate the Final output from using a tranny with the 2.76:1 spur and pinion ratio ?