gearobox internal ratio
 

my old brain can't remember how to calculate the IR of a 2wd gearbox... can someone remind me??

You need to devide the number of teeth on the diff gear by the number of teeth on the top shaft gear.

ok, thanks, wasn't sure whether you have to take into count the idler gear

As the idler gear is driven by the top shaft gear and then drives the diff gear it gets cancelled out in the calculations.