When I first started commuting on my road bike with 700x25mm tires, the distance by my odometer was around 8.2 to 8.3 miles, until I got sloppy for a spell and let my tires go soft. Then I noticed the distance was creeping up to more like 8.5-8.6 miles. When I pumped up the tires to their usual firmness, the reading settled back down. Now I am riding my old hybrid with thick, knobbly 700x32mm tires and the distance on my odometer is down to only 7.5-7.6 miles.
Odometers use pre-set wheel measurements like 26in, 27in, 700mm, etc., and have a sensor on a spoke to count wheel revolutions, then calculate ride distance and cumulative distance, as well as current speed, average and maximum speed. The wheel measurement is the wheel diameter, but the circumference of the tire (obviously) can vary quite a bit, adding up to as much as a mile of difference in seven or eight miles, whichever it is I'm really riding. I suppose that means the speed calculations aren't very reliable either.
At least now I can explain that I'm not exaggerating - with more detail than most people would want to hear. Then they'll be sorry they doubted my word.
