cyclocomputer question

[elk]

I put a cateye Strada on my bike....only problem is that the tire chart only goes up tp 700x40C
and my tires are 700 x 42 c....so I used the number for 40....just guessing at what the numbers are measuring....

Will my readings be way off? The number I need is "L" and for 40C it was 2200.

can you tell i can follow directions but have no idea what i'm doing yet???

[pll]

Cateye has a way of obtaining that calibration number. It requires measuring the distance covered by one revolution of your tire with your usual inflation and riding the bike:

http://knowledgebase.cateye.com/questions/5/My+tire+size+is+not+listed+on+the+chart+in+the+manual.++How+do+I+determine+my+calibration+number%3F

Guess it is simply a measurement of the outside circumference of the wheel+tire.

Sheldon Brown's web site has an approximation that involves using the wheel's bead seat diameter in mm (for a 700, it is 622mm): 622 + 2*42 =706. Then multiply this number by pi (3.1415...) to obtain the circumference in millimeters.

http://www.sheldonbrown.com/cyclecomputer-calibration.html#iso

Since I also have a Cateye computer, comparing the approximation with the numbers in the manual, I see that mostly it overestimates the number: for a 700x38c tire, the Cateye number is 2180 and the approximation in Sheldon Brown's site gives 2192; for 700x40, the approximation gives 2205 versus 2200. For 700x42, the formula suggests 2217... I'd try 2210 or recruit someone to help you measure!

[BleeckerSt_Girl]

Just drive a couple of different cars on a 5 mile route or so. Pick a route you like to bike regularly. If their odometers are the same you can probably count on that.
Then ride your bike on the route. If the computer mileage is more than what the cars said, then set your tire diameter a bit smaller. And vice versa. Make small changes at a time, until your computer's mileage reading matches the car odometer for that route. This can take a couple weeks of riding and adjusting, but in the end you can be sure of more accuracy than using other people's charts.

[elk]

thanks pil and lisa
I'll do both!!
thanks for looking that stuff up for me, pil.....:)
elk