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I have 700x35C tires. The manual seems to say to set the circumference at 2205. It was not written with English as the primary language, and leaves much to be desired.

I went on a ride with someone else, and her computer said we went 5.68 miles while mine says 8.85. My phone GPS more closely confirms her number (5.4 mi).

I went poking around the Internet to find the real circumference, and found two numbers: 1370 and 2168. Just doing a ratio between what my GPS stated and the the numbers on my odometer, I think the 1370 number is more accurate.

I am not looking for 100% accuracy, just something closer than 3 miles off. What number has anyone else used in this setup?
 

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The only way to do it right is to do a tire roll-out. Otherwise, you might as well leave the computer at home.
 

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A roll out test is measuring just how far the wheel travels in one rotation of the wheel. That is what the computer uses in its math to compute everything else.

I use a some masking tape. Some use grease pencils. Lots of things you can use. Put a mark or reference point on the ground, and one on the wheel so they line up. Then you push the bike forward till the wheel rotates one time with the mark on the wheel as close to the same position relative to the ground as you can eyeball. The put another mark on the ground. The goal is to get that distance one rotation makes. Measure between the 2 points on the ground. That's the distance you need.

Now it might be in inches and some bike computers want mm so you can either let one of the online converters make the conversion for you or you can multiply by 25.4. Thats the number most computers want
 
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