Commuting - 700c=x"??

I just got a bike computer, and in calibrating it I had to figure out the diameter of my wheel (I have 700x38c tires.) My spare tube is equivalent to a 28x1 1/2 tube, so I set the computer for 28". However, an internet search was unclear, and I get the impression that 700c might mean 27". Which, if any, is correct?

I don't think that a 28 x 1.5 tube is going to fit a 700 x 38 tire correctly. There should be instructions with your computer, that explain how to do an accurate roll out using a measuring tape. You should also be able to put in a circumferance rather than a straight inches measurement. Depending on the tire, a 700 x 30 should be about 2180mm circumference, so you could use that number.

Try this site yet?

http://www.sheldonbrown.com/tire_sizing.html

Sheldon Brown at the Harris cyclery site can give you the skinny on the various rim designations. 700c is also called 622 for the metric (mm) dimension of the bead diameter....all of that is essentially irrelevent to you. The bike computers that I have worked with allowed some degree of programming on the basis of circumference rather than diameter and this is measured by sitting on the bike with the tires at operational pressure and measuring how far one revolution of the wheel (that has the magnet) takes you. This is needed because one 700cX38 tire can be quite a bit different from another. And I ain't joking there! The computers I have used, had a default table that related tire designation to a given roll out circumference. In my experience the table was way off. This topic is beat to death in several other threads.

Do a "roll-out". Pump the tire with the sensor up to normal pressure. Sit on your bike against a long wall, like a garage door. Have someone put a mark on the tire where it touches the pavement, and mark the same spot on the pavement. While sitting on the bike, move yourself along whatever you're holding onto, until the mark on the tire has made one complete rotation, and is in the "down" position. Mark that spot on the pavement. Measure the distance between the two marks, and do any conversions necessary for the units the computer is set in.

I just take a measuring tape (the cloth like kind used in sewing) and just wrap it around the tire...boom! easy measurement.

I'd just punch in the 2180mm. You could be right on or, maybe, 3/4s of a mile off for every 50-miles at the most is my guess.

The actual outside diameter depends upon the size of your tire. The most accurate way is to roll it out on the ground, with your weight on the bike compressing the tire.