Re: Setting computer
You can look on-line to find the circumference for most tires but at best you get a general number that is "in the ballpark" of the number you are looking for. In my experience they are generally several millimeters off at best.
As has been mentioned, the "roll out" method is best. Personally I stretch out a 25" tape measure and line the front tire up at the zero mark with the valve stem at the 6 o'clock position. Then roll forward along the tape until the valve is once again at the 6 o'clock position. This assumes, of course, that the magnetic pickup is on the front tire. I then use an on-line conversion tool to covert the inches figure I get to millimeters, which is what most bike computers require.
Using chalk or tape to mark the outside edge of the tire would be more accurate, however, so I picked up a good idea here!