how to calculate a new speedo gear teeth count after changing tire sizes?

[1991 Blue danger ranger]

hey, im going to be putting 31's on my truck soon (currently 235/75 r 15, about 29") and want my speedometer to remain accurate after the change, and i know that i have to change my speedometer gear to do that. i want to use this https://www.therangerstation.com/tech/changing-the-speedometer-gearing/#gsc.tab=0 but its only based on changing your diff gear ratio, not tire size. any help is appreciated!! (or honestly if you've done the same job that also works lol)

 

[scotts90ranger]

I would just figure out which gear you have now and get one with one tooth less then check with GPS or something, you're likely going to be pretty close, that's not a huge change in tire size.