in tech archive there is a speedo pinion chart click it then click on auto trans and type in tire size and gear ratio to get tooth count needed.

28 inch tire?

I used a 225/75/15 tire size = 28.2 x 3.23 = 30 tooth pinion

sound about right as my M/Ts are 29x15x15 = 325x50x15 = 28" tall and I run a 3.23 with 30 tooth seems dead on in mine.

if your 28" is on tall side them a 31 tooth is next.