# 700x23 tire circumference for bike computer



## e_rat

I searched the forum and did not find the answer... Anyway, here is my question:

What is the circumference in mm for your 700x23 tire? 

The computer default is 2096mm. I looked up online and someone suggests to do the actual measurement for more accurate calculation, so I did. My tire is 2130mm. Then I looked up online again, and someone measured 2110, 2130, and 2133. I tend to use 2130, and of course I will do a experiment, but what to compare against to check the actual reading?

What will you use? 2096 or actual measurement?

Thanks!


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## cxwrench

e_rat said:


> I searched the forum and did not find the answer... Anyway, here is my question:
> 
> What is the circumference in mm for your 700x23 tire?
> 
> The computer default is 2096mm. I looked up online and someone suggests to do the actual measurement for more accurate calculation, so I did. My tire is 2130mm. Then I looked up online again, and someone measured 2110, 2130, and 2133. I tend to use 2130, and of course I will do a experiment, but what to compare against to check the actual reading?
> 
> What will you use? 2096 or actual measurement?
> 
> Thanks!


You measured your tire and came up w/ a number, why would you use something else? Don't overthink it, measure the tire...enter the number...ride your bike.


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## Randy99CL

The stupid thing about this is that there is really only one right way to measure it and I'll bet you got it wrong.

You have to have the tire on the bike, inflated to the normal pressure.
Put a big wet dab of paint on the tread of the tire and ride dead straight for 20 feet or so.
Measure from the first dot on the pavement to the last and divide it out.

And no, I am not anally retentive enough to have done that.


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## cxwrench

Randy99CL said:


> The stupid thing about this is that there is really only one right way to measure it and I'll bet you got it wrong.
> 
> You have to have the tire on the bike, inflated to the normal pressure.
> Put a big wet dab of paint on the tread of the tire and ride dead straight for 20 feet or so.
> Measure from the first dot on the pavement to the last and divide it out.
> 
> And no, I am not anally retentive enough to have done that.


Surprisingly there are easier ways to do this that don't involve paint.


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## Randy99CL

cxwrench said:


> Surprisingly there are easier ways to do this that don't involve paint.


Yeah, I thought of one after I posted.

Obviously, it is a matter of measuring the squish.

Sit on bike, someone measures from center of axle to ground X 3.1415926?


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## cxwrench

Put a mark on the ground. Line up the valve w/ the mark. Sit on bike, roll forward 'til valve has made exactly one revolution, make another mark. Measure.


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## e_rat

Thanks for th inputs! But I am not having problem measuring it. I have a measuring tape and measured it very carefully. 

I will enter 2130mm to my bike computer. That is 34mm more than default value. Is it 2096 a standard, or an average?

Also, how do I compare it against for checking the accuracy? Strava? MapMyRide? With my iPhone?

btw, my computer is CatEye Wireless Strada. Thanks!


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## looigi

I haven't found squish to matter. I compared rollouts both weighted and unweighted and the difference as less than the few mm accuracy to which I measured.


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## SauronHimself

You know the radius of a 700c wheel. At your normal operating pressure measure the height of the tire from the top of the rim to where the tire ends. Add that to the rim's radius and then 2*pi*r.


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## looigi

A rollout is easy and more accurate, IMO.


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## Randy99CL

looigi said:


> I haven't found squish to matter. I compared rollouts both weighted and unweighted and the difference as less than the few mm accuracy to which I measured.


You're right. Two mm of squish only makes 6.28mm difference in the circumference.

I got the method I first posted from checking automotive tires where there can be huge differences between manufacturers, models and inflation.

Edit: I was surprised when I first got a GPS for my car that at 85mph on the speedo the GPS says I'm only going 80. 
The tires are the recommended stock size so when I replace them I may go slightly smaller. And wider, naturally.


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## vmps

e_rat said:


> Thanks for th inputs! But I am not having problem measuring it. I have a measuring tape and measured it very carefully.
> 
> I will enter 2130mm to my bike computer. That is 34mm more than default value. Is it 2096 a standard, or an average?
> 
> Also, how do I compare it against for checking the accuracy? Strava? MapMyRide? With my iPhone?
> 
> btw, my computer is CatEye Wireless Strada. Thanks!


Manufacturer tire size ratings have essentially no relationship to reality. Measure and be done with it.


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## c_h_i_n_a_m_a_n

My cateye wireless also says 2096mm in the manual. Just go with what you measured.


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## PlatyPius

cxwrench said:


> Put a mark on the ground. Line up the valve w/ the mark. Sit on bike, roll forward 'til valve has made exactly one revolution, make another mark. Measure.


This.
Just enter the damn number and stop analysing the hell out of it.


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## Jay Strongbow

e_rat said:


> Thanks for th inputs! But I am not having problem measuring it. I have a measuring tape and measured it very carefully.
> 
> I will enter 2130mm to my bike computer. That is 34mm more than default value. Is it 2096 a standard, or an average?
> 
> Also, how do I compare it against for checking the accuracy? Strava? MapMyRide? With my iPhone?
> 
> btw, my computer is CatEye Wireless Strada. Thanks!


Just go ride your bike for cripes sake. you'll find a 40 mile ride is very similar to a 39.99 mile ride so I wouldn't worry about accuracy as long as you have a ballpark setting.

If you want 100% pinpoint accuracy you'll need to reset it every ride and during the rides. Unless you have a pump that can pinpoint a certain exact and consistant air pressure every time and your tires don't lose air or expand/contract because of temperature changes during a ride. If you figure that out then you'll only have to reset it for tread wear. 

Seriously dude, get over this. If you must be anal about something to do with cycling there are bigger fish to fry. Worry about your fit maybe. That should keep you busy for a while.


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## Kerry Irons

e_rat said:


> What is the circumference in mm for your 700x23 tire?


Mine is 2094, but I don't use a computer so that's meaningless. Use what you measured and then go ride someplace that has mile markers along the road (don't use trail mile markers unless they're left over from a railroad). Those mile markers on the road are put in place by surveyors and are very accurate. Compare your computer reading over 10 miles (or more) and then adjust your calibration if necessary.


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## rruff

e_rat said:


> Thanks for th inputs! But I am not having problem measuring it. I have a measuring tape and measured it very carefully.
> 
> I will enter 2130mm to my bike computer. That is 34mm more than default value. Is it 2096 a standard, or an average?


You are getting lots of bad advice. A GP4000S is tall for a 23mm and it's only 2090 on an XR270 rim. A 20mm Supersonic is only 2060. I doubt that your tire is really 2130 in circumference. And even if it was, you'd have to subtract the amount that it flattens. The radial amount is ~3mm which will reduce the circumference by ~19mm.


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## rruff

looigi said:


> I haven't found squish to matter.


Should be at least 15mm different if you did it right.


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## c_h_i_n_a_m_a_n

Come to think of it my 25mm GP 4 Seasons, has a circumference of 2107mm.

I marked the floor and rolled the bike with me on it in 1 revolution. Should be pretty close. Did it a couple of times in different places and just used the average.


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## looigi

rruff said:


> Should be at least 15mm different if you did it right.


Is that the difference you measured in weighted vs unweighted rollout, or calculated from the change in radius? I'm talking about measured rollout.

I find that the change in rollout is much less than what you'd calculate from squished radius. I figure this is due to tire squirm.


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## rruff

Rollout... and change in radius. I don't see how tire squirm is going to make a difference.


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## looigi

OK. It's been something like 12 years since I compared weighted and unweighted rollouts, so I went out and tried it. I got 13 mm shorter rollout out of 2120mm for the rear tire with 170 lbs in the saddle on a 23mm tire at 100 psi. 12 years ago when I last checked it was 160 lbs, 110 psi, and the front tire rather than the rear, all of which would add up to a smaller difference between laden and unladen rollout.


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