# How to determine proper wheel size?



## tyson8earzz (Jul 7, 2004)

I bought a Garmin Edge 305 computer bundle with cadence and HR option. I mounted everything and it works really well. I love it but I question the accuracy of the speed / Max speed.

Under Settings > Bike Profile, i see the option to insert the wheel size but currently it is on AUTO. But under that it says 2174mm. If i have a 700x23c mounted tires and standard size road bike wheels, what is the proper milimeter size supposed to be? How can I make sure the 2174mm is correct?


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## Kerry Irons (Feb 25, 2002)

*Measure it*



tyson8earzz said:


> IUnder Settings > Bike Profile, i see the option to insert the wheel size but currently it is on AUTO. But under that it says 2174mm. If i have a 700x23c mounted tires and standard size road bike wheels, what is the proper milimeter size supposed to be? How can I make sure the 2174mm is correct?


The standard technique is to do a roll-out, where you measure how far your bike goes (when sitting on it) with one revolution of the wheel. This will vary with your weight and tire pressure. An alternative is to compare the computer readout with a known measured course of at least 10 miles. Examples would be using road mile markers. Then you can correct for the difference. For me, a 23 mm tire rolls out to 2095 mm.


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## Cory (Jan 29, 2004)

*Just in case, here's a definition of "rollout."*

Kerry's right, as usual. Just in case you're not clear on "rollout," though, it means one complete revolution of the wheel. It's most accurate if the tire is pumped to the pressure you'll be riding and if you're sitting on the bike, and usually if somebody supports you (or you can have one hand on a wall or something) so the bike goes absolutely straight, no wobbles. If you're really picky, you can mark the tire and floor with chalk and go forward until the chalk mark touches down again, then measure the distance in millimeters, which will be your setting. As a practical matter, though, missing it by half an inch isn't going to make much difference..


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## JCavilia (Sep 12, 2005)

*If measuring rollout is inconvenient,*

You can get pretty close with a calculation using the rim diameter and tire width, but to make it even easier, here's a table of common sizes.

http://www.sheldonbrown.com/cyclecomputer-calibration.html#ccc (column "F" is circumference in mm).

It looks like your 2174 mm setting is probably too high, by about 3.5%. That would mean if it says you hit 50 mph on that wild descent, you were really only going a bit more than 48.


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## wim (Feb 28, 2005)

*Radius works for me.*

For me, getting a good wheel radius reading works well. With you on the bike in a riding position, have someone measure from the floor to the exact center of the skewer on the non-QR lever side of the wheel. Circumference is Pi times diameter, so multiply your radius reading by 2, and the product of that by 3.14.

For example: the radius of my rider-loaded front wheel with a 23 mm tire is 342 mm. That times 2 = 648 mm wheel diameter. That times 3.14 = 2,147.76 mm wheel circumference, rounded up to 2,148 mm. Your numbers will be a bit different, I'm sure.


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## Dave_Stohler (Jan 22, 2004)

A quick way to estimate diameter is:

622mm * (2* tire height) = Diameter

The tire height is, essentially the "width" of the tire (originally, that number was the height, but things have changed).

Take that number, and multiply by π.


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## lawrence (May 17, 2005)

*Mine is 213cm*

My wheel size is 213cm. I used a cloth tailors tape and measured the wheel diameter off the bike but with full pressure. If you sit on the bike and measure the wheel, the flex in the tire in that one spot, I don't think will much of a difference in the diameter.


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