Well, I originally planned on using the Enduro 8 speedo and actually bought one, however I then ran across a Panoram 17 function bicycle computer. The nice thing about it is that it displays 4 separate functions at all times, so you can be viewing your current speed, current time, current ride-time and odometer all at the same time without touching a button. I'm now in the process of figuring out a way of mounting it since it is supposed to be mounted on a horizon section of the handlebar and I don't have that part of my handlebars available. I can't wait to get it installed because it seems like a quality piece of equipment.
As for your tires, on digital speedometers you measure the circumference of your tire and then you enter that number (usually in millimeters) into the computer. It calculates everything automatically from there.

JC