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Most cycle computers have a means to enter tire circumference so as to obtain accurate speed and distance metrics.
The best way to check tire circumference is to sit on the bike and roll along next to a long measuring tape:
- Point the tire valve at the “0” mark on the tape measure.
- While keeping all your weight on the seat in a semi-normal riding position, maneuver the bike in a straight line along the tape.
After four (4) revolutions, I get the distance to the 1/2", then divide by 4 and convert to millimeters.
GPS-based systems don’t need tire circumference, but in my experience they are very inaccurate for riding where there are close-linked turns and/or heavy tree cover (e.g. mountain biking). I always prefer a bike computer that gives me speed and distance by a speed sensor; I have observed ridiculous errors with GPS, especially for speed (e.g. 3 mph instead of 8 mph!).
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I measured these values on my road bike:
Veloflex Record tubular 700 X 22C @ 120 psi: 2098 mm Schwalbe Ultremo ZX 700 X 28C @ 90 psi: 2124 mm
The difference is small at only 1.2% for the 700 X 22C vs the 700 X 28C, but that is 1.2 miles during a century, which becomes more and more significant if navigating turns denoted by a mileage mark. And if the turns are only 1/4 mile apart, even at the 25 mile mark, the error exceeds the gap between the turns! This can be very confusing in a city area with turns every 100 yards. So it is worth being accurate.