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Old 05-30-12, 11:56 AM
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Derailed
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Join Date: Nov 2004
Location: South Bend, IN (U.S.A.)
Posts: 473

Bikes: Priority Continuum Onyx; Hunter CX

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The distance your computer measures is just the product of the number of wheel revolutions it counts and the circumference you input (also multiplied by a conversion constant to get from mm to miles because you're using different units):

d = n x C x const. (where d is measure distance, n is number of revolutions, C is circumference, and const. is the constant to convert the units of your circumference measurement to the units you want for your distance measurement)

If you trust the distance of 12.21 miles measured with your 700x25 Ultra Race tires, and you want to make your measurement with the 700 x 28 Ultra Sport tires get the same reading, you just need to adjust the circumference measurement up by the same percentage that you're short in distance:

Again, assuming you trust the 12.21 mi measurement, and your Ultra Sport measurement was exactly 1/4 mi short, it is short by 0.25 / 12.21 = 2.048%.

So, you just need to increase the circumference value for your 700 x 28 Ultra Sports by this same percentage: 2149 x 1.02048 = 2193.

I do note, however, that that's pretty big for a 700 x 28 tire!

Personally, I prefer to do my own roll-out measurement rather than trust the included tables (although, honestly, the tables usually work pretty well). I use a 75 foot tape measure and try to measure seven or eight revolutions with me sitting on the bike to spread the inevitable measurement uncertainties out as much as possible. And, to make the calculation super easy, I use a Google spreadsheet set up so I just enter the total distance and number of revolutions, in feet and inces, and the spreadsheet displays the circumference in millimeters.
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