According to Brewer's, I need a 31 tooth for 3.23s & 28" tire
According to the Interweb, a 32-tooth.

I have a mint 32, a mint 39, and a used 31 with a few teeth knocked off, of course

What's 1 tooth worth in MPH on this setup?
FWIW, I tend to drive by the tach on the highway, but around town I'd like to know my speed.