OK. As compulsivebikr says, this is if you want to be super-accurate.

You want to get the exact rolling circumference of your front tire, and enter the number on your computer. Each computer has a different means of entering the figure.


Find a nice smooth surface (like a garage floor or the sidewalk). It helps to draw a string along the surface so you can roll the bike straight, otherwise a wobbly path will give you an inaccurate reading. Just stretch it out and tape it down at both ends. You need about 10 feet of string for a single revolution of the wheel -- but you'll get the super-accurate results if you roll out two revolutions, so use 20 feet of string.

Make sure your front tire is pumped up to normal pressure (what you'd use on tour).

Put a piece of white tape on the outside of the tire at some point (say, at the valve stem). The piece of tape should touch the pavement but be visible as you straddle the bike. Put a vertical mark on the tape so you can precisely identify the spot where it touches the pavement. At that spot on the pavement (at one end of the string), put a mark. You want to hit it exactly where the tire-mark is vertical; otherwise it can be off a few mm and this will add up over a ride.

Get on the saddle. You'll want to "crab" down the pavement, along the string, with all your weight on the bike (this compresses the tire, which you need to do if you want to get an accurate measurement of rolling circumference). Go two revolutions (for better accuracy), and stop when the tire-mark is exactly vertical again. Mark this location.

Measure from start to end; divide by two if you've done two revolutions.

If you're super-anal-accurate, you could do this 3 or 4 times. Believe me, you'll get different results each time. Average 'em. Done.

-- Mark

Edit: Oh, one more thing! Write the number down on a small piece of tape, with indelible ink, and stick it to the back of your cycle computer. At some point on your tour, the batteries will die or the buttons will get jabbed and you'll have to enter it again. I can never remember the circumference setting more than a week or so....