I did notice near the end of my tour that the cyclometer was showing a difference of 3 to 4 percent from the GPS unit while the beginning it was around 1 percent. I think it was the result of my tires wearing down and thus making the cyclometer more inaccurate. Air pressure probably also affects cyclometer accuracy, especially on mountain bike tires. The less pressure, the more tire that is constant with the road.

I probably should have done a wheel circumference measurement to bring the cyclometer back to being totally accurate but when you're riding 70 miles a day, it's not a real high priority.

Anyhow like I said with cyclometers being so cheap, no law says you can't have both a cyclometer and a GPS.