How to calibrate speedometer?

Pretty sure mine are stock 2.71(I think that's the ratio, the lowest)

Wheels (I believe) are 5.5" wide.

Whoops you meant diameter! Any idea how I could find that at work without going out and measuring?

editx3:

Found this:
http://1970mgr.org/_Tiresizes.htm


How easy is it to switch? Where is the gear located, exactly?