@dreamer_ I was thinking the same but for another reason. I see that the loop looks like a trail with many little stones and other tiny “obstacles”. In this case the wheel will slightly goes up and down because of the obstacles and measure more than the “flight” distance. You could think that this is negligible but I think it is not the case. To asses it, I have computed the ratio between a straight line and an undulating line where every 10cm there is a difference of 1cm of height. The ratio is 2.4%. So, far from negligible and close to the difference found between the watches and the wheel. Unfortunately, I don’t have a measuring wheel to test the hypothesis.