For cyclecomputers that require a diameter value, divide the result by 3.1416 (ПЂ), and for those which require a radius value, divide the result by 6.2832 (2 x ПЂ). If the tape measure is divided in inches, multiply the measured circumference by 2.54 for centimeters or 25.4 for millimeters. You may measure for one wheel revolution, or for greater accuracy, for three or four - whatever your tape measure can span - and divide by the number of revolutions. Otherwise, the wheel may not follow a straight path.

With either approach, the rider must hold the handlebars straight while an assistant balances and pushes the bike. You may use the valve stem as a reference, starting the roll with the valve right over a perpendicular line, and ending when the valve is back at its low point.ДЄnother approach is to put a small dot of paint on the tire and measure the distance between the marks that the paint prints on the road. It is possible to do a roll-out test while lightly scooting along while bearing weight on the handlebars and one foot on a pedal, but it's better to have an assistant holding the bicycle upright and pushing it along. Since the effective tire size is affected by tread thickness, tire pressure and rider weight, the rolling circumference should be measured by rolling the bike with the rider aboard. The rear wheel 'creeps' on the road surface as you pedal, and can skid during braking, so it gives a less-accurate readout. Unless you need to count 'miles' ridden on a stationary trainer, it is best if you measure the roll-out of the front wheel and mount the computer sensor there. If you require more accuracy, you can do a 'roll-out' test. Values read from a chart or derived from ISO/ETRTO numbers will generally be accurate to within one or two percent, which is good enough for most cyclists, and more accurate than most automobile odometers. If your marked tire size falls between two sizes shown on the chart, interpolate the appropriate calibration number between those above and below, or for greater accuracy, do a roll-out test (keep reading.). The chart below doesn't list all possible tire sizes, but does list the most popular ones. The table below is based on rim sizes and tire cross-sections rather than than nominal sizes. It is less than 19 inches in effective diameter. For example, a 44-406 (20 x 1.5') tire is not really 20 inches in diameter. Tires which a national standard identifies by outside diameter actually vary depending on the tire cross section. Group AДifferent manufacturers have used different brands of tires to calibrate - or have done calculations based on nominal dimensions - so there can be a slight inconsistency between the numbers in the tables and the most accurate number for your bicycle. We have classified cyclecomputers into six groups according to the number used in calibrating them.

We have provided tables of tire sizes for this purpose. The simplest method is to set the wheel sensor for a nominal tire size.

Echo t3 cycle computer manual