Originally Posted by TheRCF
If it can be done, I prefer more accuracy over less accuracy. If I do a century, I want to be sure it is really a century. Other people I ride with often talk about how fast they did this or that ride. Just this week, I found one of them was likely 10% off! So, if he was going 20 mph, he was really going about 18 mph. I don't know about you, but I find a speed increase of even a couple of 10ths to be pretty significant progress, so it is important to know what your numbers really are.
Given that your most precise source for distance measurement is a tape measure and that you are measuring, at best, a few revolutions of your wheel, the error associated with this measurement is probably going to be, at worst, 5mm for a single revolution. 5mm error for a 2100mm circumfrence wheel comes to 0.23% error. On a ride of 100 miles, this equates to an error of +-0.23 miles. So, if you had this uncertainty in your setting, you can simply ride an extra 1/4 mile and you will know that you have ridden at least a full 100 mile century.
However, this does not tell the whole story. There are other error sources that have not been addressed. First off, when you do your rollout test, unless you have a helper push the bike while you are on it, chances are you will not be in your exact riding position. This will result in a different weight above your wheel causing some amount of additional uncertainty in the measurement. Second, even if you are able to get into position, you are unlikely to ride the same position during you entire ride. You might stand up for some climbs. You may sit back some or ride in the drops at times. all these chance your weight distribution and effective wheel circumfrence.
Don't forget that the road has bumps which will bounce your tire and cause momentary changes in effective circumfrence. These may or may not cancel out, I don't know.
Keep in mind that you may not ride the same line as someone else. Depending on how straight you hold your line and what line you take through the curves, your computer will show a bit more, or less than someone else's, and perhaps more or less than the published distances for a ride. When you stop for a break, you may not stop your bike right on the road and add a few hundreths of a mile riding through a prking lot or two.
Finally, most bike rides are informal affairs and any published distances by the organisers should be taken with a grain of salt. The organisers probably either used some mapping software to compute their route distances or drove the route in a car. Either case is highly prone to inaccuracy.
The bottom line? Use your best judgement and don't worry about it. There is no magic to the 100 mile mark. Whether you do 98 or 102 miles, you're going to get the same benefit and feel the same soreness afterward. Remember, your computer will never precisely match mine; and as I stated earlier, my computer is absolutely dead-on perfectly accurate.