With the OD you give, there's a good chance it's a 1 module gear. (fwiw: since 1 module is 25.4 DP, the PD should be 1.5748")
afa: pressure angle...
Check the base pitch -- Clive pointed you in the right direction on this.
Use a vernier and check over something like 4 teeth, and 5 teeth... and look at the difference. It's important to make sure the vernier jaw faces are on the flanks (profile) of the teeth on both measurements (not touching the root of the tooth or the tip).
If it's 20° pressure angle, you should see about .116" difference between measuring over 4 teeth and 5 teeth, (or between 5 teeth and 6 teeth).
If it's 14.5° pressure angle, you should see about .1197" for a difference.
If it's 25° pressure angle, the difference should be about .1121".
If it's anything relatively modern, it's unlikely that it's 14.5° PA (unless based on a very old design).
Here is a DXF of a 40 tooth, 1 module 20° gear (you'll probably need to right-click on that link and do a "Save As" to save the DXF on your computer).
If it turns out to be metric, here is a source for the cutter:
http://www.newmantools.com/cutters/gear.htm#mod
PM