Speedometer gear?

[bad68bowtie]

I'm gonna be putting some new tires on the truck hopefully in the next day or so and I'll be going down a little bit in size. I had written down somewhere how to figure up how many teeth you need on the speedometer gear but of course now I can't find it. Does anybody know how to equate this or have a link to figure it up? If one of y'all knows how to do this offhand I'm gonna be running 255/60/15 tires with a Turbo 350 and a 3.08 rear end. Thanks.