Originally Posted by hokiefyd

Congratulations on your purchase! Regarding the circumference for your computer, and sitting vs. rolling, it would change it some, depending on your weight and inflation pressure. You could measure this pretty closely, I think. Measure how long on the ground one unloaded rotation is. Line your valve stem up on the ground, mark that spot, roll the bike forward until the valve stem is back at the ground, mark that spot, and measure. That's your unloaded circumference (and is probably around 2300mm). For a loaded circumference, do the same exercise, but roll the bike with you on it. It may help to lower the saddle so you can have one or both feet (or at least toes) on the ground to move you forward. Be sure to lean forward on the bars as you normally would riding (rather than just sitting bolt upright on the saddle). Start with the valve stem at the same spot as before, and roll forward until it's back on the ground. In theory, that distance would be just a little bit less than before.

If so, your odometer is likely reading just a hair more than you've actually traveled. If one unloaded rotation is 2300mm, then 100 rotations equals 23 meters, and that's what you'd have in your computer. Now, if a loaded rotation is only 2295mm, then 100 rotations equals 22.9 meters (but your computer will still read 23 meters for the 100 rotations). That notional example is 0.4% error, so an indicated 40km ride might really be 39.8km. I'd say there's that much error in the actual measurement of the computer itself, and it's probably not worth worrying about. That said, I would be interested to know if you do measure the difference between loaded and unloaded, and what that might be if you do.