# Tire Circumference.



## DS1239622 (Mar 21, 2007)

I recently switched from Michelin Krylion Carbons to Conti GP 4000S's and upon doing so saw the mileage on my 'staple' rides drop across the board. Obviously this is due to a circumference difference between the Michelins and the Conti's. I was using the standard 2096mm for a 700X23 tire for both tires. Both tires are 700X23.

Following the advice of threads on here I tried to measure the circumference of the tire by placing a small dab of peanut butter on the center of the tread and rolling the tire (along as straight a line as I possibly could) for one revolution and then measuring the distance between peanut butter marks.

Doing so got me a distance of 2120mm. I then rode some of my standard rides and saw the mileage increase slightly on the Conti's versus what I was getting from the Michelins. Also just from a visual observation it does appear the Conti's are taller (which would add up to a larger circumference than the Michelins).

My question is does the 2120mm seem reasonable or too large? Any Conti GP 4000S owners out there try this? Im trying to determine which is more accurate, the 'benchmark' I was getting with the 2096mm and the Michelins or the now slightly longer distances I am seeing with the Conti's and my measured 2120mm.

Thanks!


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## JCavilia (Sep 12, 2005)

-No experience measuring those tires, but those differences are in the range of reasonable.

-If I'm doing the math right, a tire that's about 3mm higher in the middle (due to either casing size or thicker tread or both) would account for your difference. That's not unlikely.

-did you ever measure the old tires? You may not have been using an accurate figure for those. So it's hard to say whether the old or new is more accurate.

-the difference is about 1%. IMHO not a large deal unless you're measuring a TT or something.

-I'd try doing the measurement a couple more times, on different surfaces. And try it with normal weight on the bike, if you didn't before. The deflection of the tire under load can make a little difference.


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## DS1239622 (Mar 21, 2007)

Thanks for the reply. I will try measuring a couple more times. Its not a real big deal, im just a bit anal and like to be able to accurately compare my old times with my new ones to mark improvement etc.


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## Kerry Irons (Feb 25, 2002)

*Real number*



DS1239622 said:


> Thanks for the reply. I will try measuring a couple more times. Its not a real big deal, im just a bit anal and like to be able to accurately compare my old times with my new ones to mark improvement etc.


My measurement of a Conti GP 4000 23mm tire gives a circumfrence of 209.4 mm. However, you might experience a different number depending on how hard you inflate them. If you want to be really accurate, find a 10 mile (or longer) stretch of road where there are mile markers. Ride the distance and then compare your computer reading at any given calibration. Adjust the calibration number up or down by the ratio of what your computer reads vs. the actual distance. Those mile markers are measured by surveying, which is very accurate.


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## SwooshDaddy (May 8, 2009)

You could try using mapmyride.com and compare the difference, I think the site is pretty darn accurate.


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## jmlapoint (Sep 4, 2008)

I've used them both and they measure about the same.


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## Fixed (May 12, 2005)

*G P S?*



Kerry Irons said:


> My measurement of a Conti GP 4000 23mm tire gives a circumfrence of 209.4 mm. However, you might experience a different number depending on how hard you inflate them. If you want to be really accurate, find a 10 mile (or longer) stretch of road where there are mile markers. Ride the distance and then compare your computer reading at any given calibration. Adjust the calibration number up or down by the ratio of what your computer reads vs. the actual distance. Those mile markers are measured by surveying, which is very accurate.


How accurate are GPS computers to calibrate? Borrow one?


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