Resetting a computer I use on two bikes after battery replacement, I couldn't find where I'd written down the wheel size, so I rolled out the front wheels from each bike.
These bikes both have 700X28c tires. I should be able to swap the computer head between the two of them without making any adjustments, and I thought I could. But when I rolled out the wheels I discovered one rolls out a fat centimeter longer than the other one. Like 1.3-ish centimeters.
A centimeter? That's all? That's huge. Over a ten-mile ride that adds up to a cumulative error of almost 249 feet.
According to the handy chart provided with new Sigma computers, a 700X28 should roll out to 215 cm. Neither of these rolls out to 215, reaching 212 and 213-plus, respectively. The chart also says a 700X32 should roll out to 217, but my 700X32 bike rolls out to 218. And I want my centimeter!
Tires of the same nominal width can and do have different heights. Height affects circumference. Circumference determines rollout. And inflation, or lack thereof, can further alter the result. The 28 on my road bike is taller than the one on the fixed gear with which it was sharing a computer. Even so, it falls short of the 215 offered by the Sigma chart. That's freshly inflated to its rated pressure of 105 psi, so it's as tall as it is going to get.
Varying tire height can also complicate fender installation on tight frames, or even tire installation on really tight frames. For the record, the tall 28s on my road bike are Panaracer Paselas. The height makes them very comfy on bumpy roads. It also keeps them from fitting some frames and forks technically roomy enough for them, as I have discovered when fitting wider tires to some customers' road bikes. In one case, the tires they were replacing were marked 700X28, but were barely 25s. They dated from the super-skinny tire era in the late 1980s.
When setting your computer, have no doubt: roll it out.