# Quick question about entering tire size for cycle computer

Discussion in 'Cycling Equipment' started by LS17, Aug 12, 2005.

1. ### LS17 New Member

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I just bought my 1st cycle computer (after getting my 1st bike a few months ago) and it asks me to input a number for the tire size. It shows something like 2024 and I have to change it to get an accurate speedo reading. All I know is I have 700x23 tires by looking at the markings. Anyone know what the correct # to input should be and how to get that #?

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2. ### gclark8 New Member

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Sounds like the circumference of the tyre in millimeters. Measure it, mark the floor at the start, wheel the bike for one revolution of the wheel and measure the distance.

3. ### NSM3 New Member

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Try 2096 for a 700c x 23

4. ### Arathald New Member

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Much more acurate way is multiply the diameter of the bike in mm (700) by pi (~3.1416), which comes out to 2199

edit:
On further investigation, maybe you want to use this method instead: http://www.bobwheeler.com/bicycle/Computer/computer.html

5. ### RickF New Member

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Depending on the tension of the spokes, brand of tire, the air pressure in the tire, and the weight of the bike and rider, the actual diameter will vary from the nominal 700 mm. Using the roll out method, one revolution on my bike is 2190 mm, which would make the effective diameter 697 mm, even though it is labeled as 700 mm. According to Specialized, the recommended setting for that tire is 2192.

The bottom line is if you want to be accurate, you need to measure. On the other hand, the difference between 2190 and 2192 mm per revolution of the wheel is about 57.8 inches per mile or 0.09128%. I can live with that. On a measured track, that would be less than the difference between riding the inside of the lane versus the outside of the lane.

6. ### NSM3 New Member

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I was just quoting my Cateye manual;

2096 for a 700 x 23c
2086 for a 700 x 20c

I assume they have done enough research to tabulate the numbers accurately enough?

7. ### penguinonabike New Member

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now this may seem a silly question... but...

did you read the instructions they come with a chart on there telling you the number to put in... at least the one i fitted on mine did anyway...

but if i remember it is the circumference

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