# What's the outer diameter of a 700x23c tire?



## ph0enix

I want to do my own gear ratio/speed calcs.

Thank you!

J.


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## RHankey

Depending on the make, model, and batch of tire, the actual rim to which it is mounted, not to mention rider weight and tire inflation, there will be no standard runout.

You could measrure the actual runout in less time than it took to type your short question.


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## ph0enix

So based on what you're saying my computer could be anywhere from a little off to way off depending on the tires I use and the other factors you mentioned.

Thanks!


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## RHankey

The % difference between a 19mm and a 25mm road tire isn't going to be huge. But if you are concerned enough to be asking the diameter, then the only right answer is to measure the rollout.


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## laffeaux

In theory it's 700mm, but as RH says, it varies.

The differences aren't that great. The circumference of your wheel is about 7 feet. Even if you're measurement is off by a 1/2 inch you're still within about 0.6%. You should be able to measure more accurately than that.


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## Fixed

*26.6"*



ph0enix said:


> I want to do my own gear ratio/speed calcs.
> 
> Thank you!
> 
> J.


Mine measure out to 26.6", which is the rollout / pi.

Here is a spreadsheet calculator: http://www.midcalracing.com/gears.xls


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## rruff

My rollouts with 23mm tires have ranged from 2080mm to 2096mm.


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## Kerry Irons

*What theory?*



laffeaux said:


> In theory it's 700mm.


Eactly what theory is that? A typical 23 mm tire will have a diameter of 667 mm, or a circumfrence of 2095 mm or 210 cm (the number you typically put into the computer). As others have noted, tire brand/model can affect this a bit, and it's easy to do a rollout measurement. 

If you want to be really accurate, find a highway with mile posts and compare your computer's reading with posts over a 10+ mile distance. The mile posts are measured by surveyors and are generally VERY accurate.


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## ph0enix

rruff said:


> My rollouts with 23mm tires have ranged from 2080mm to 2096mm.


Mine came out to 2191mm which is not adding up because I took a reading off the computer today while riding. I was spinning at exactly 100 RPM in 50/12 and it displayed 32.5 Mph suggesting 2089mm (665mm diameter). When I measured the diameter of the wheel with a tape measure, it was just about 26.5inches (673mm) so my roll out seems to be the least accurate.


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## rruff

The tire squashes down a fair bit so you have to do a weighted rollout.


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## ph0enix

I'll do another rollout under load. Also, there is a 400m running track in the park by my house. I'll ride along a lane and see if the computer ticks 1/4-mile for every lap.


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## fallzboater

The easiest/best way to calibrate, IMO, is to also carry a GPS on a relatively straight, flat ride, then adjust the cyclecomputer number by the percent the odometer is off.


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## ph0enix

My GPS is only accurate to 60ft so it does not make for a good measuring device.


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## fallzboater

ph0enix said:


> My GPS is only accurate to 60ft so it does not make for a good measuring device.


I find my cheap Garmin eTrex to be better than 20ft, with a decent horizon. Even if you use 60 ft, that's only a little over 0.1% error over 10 miles.


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## Kerry Irons

*Better technique*



ph0enix said:


> I'll do another rollout under load. Also, there is a 400m running track in the park by my house. I'll ride along a lane and see if the computer ticks 1/4-mile for every lap.


Your bike computer has a "running algorithm" and so won't be very accurate at all for such a short distance. You need a measurement where the known error is less than 0.5% if your calibration is in cm, and less than that if your calibration is in mm. An example of such a measurement is the example I gave above: find a highway with mile posts and compare your computer's reading with posts over a 10+ mile distance. This will give you what you need (0.05 miles out of 10 = 0.5%).


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## laffeaux

Kerry Irons said:


> Eactly what theory is that?


The "theory" that the rim that it's mounted too is called a "700c" because the tire that is mounted to it it supposed to measure out at about 700mm, much like a 650b is supposed to be 650mm, and a 26-inch is supposed to be 26 inches. None of these measurements are terribly accurate but that's where the names come from.


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