'65 c10 - I'd like my speedometer to read correctly

[Woogeroo]

my '65 stepside c10 lwb(127"wheelbase)

has a 350 sbc engine

a th350 transmission

and a stock rearend with about the 3.08 - with the posi/limited slip.

the plastic gear inside the tranny that attaches to the cable that goes to the back of the speedometer has 23 teeth on it.

I have not yet quite figured out the math on this to figure out what I need to do. I know the number of teeth on that plastic gear has something to do with it.

My truck also has the stock type 15 inch wheels.

With these tires:

P255/70R15 uniroyal gts

Now, is there something in the back of the speedometer that needs to be changed also?

Any help or moral support is appreciated.

-W