with the 3 speeds being 0.65:1, 0.85:1,and 1:1 as best I can tell. PTO shaft sprocket is a 16 tooth, what number of teeth should I run on the rear axle?
Best you can tell might not be good enough.
I assumed these engines might be geared liked something below.
Primary? 3:1?
1st 3:1 ( 9:1 total)
2nd 2:1 ( 6:1 total)
3rd 1:1 ( 3:1 total)
if you are inverted, 0.65 is about 1.54 :1
0.85 is 1.18 :1
I have no clue what your primary reduction is and those gear ratios seem suspect to me. If you can find out what the true sprocket counts are in the gear box I could provide better suggestions.
going from 18” diameter tires to 6” diameter tires reduces circumference to 0.333 of the original size. To keep ratio consistent, you would need to decrease the rear sprocket tooth count (from the original vehicle) to have 1/3 of the teeth. For example: 16: 48 with 18” tires is the same as 16: 16 with 6” tires.
Depending on weight and desired speed and or need for torks, you probably want to adjust.
Little slick tires and too large of a sprocket might have you doing burnouts and or wheelies.