Originally Posted by tomato coupe

Here you go ... 10 rides on the same route, logged on a Garmin Edge 520 without a wheel sensor. The starting point probably varied by about 20 feet, and the finishing point by about 5 feet. If you dig into the files, you can extract a couple more digits for the distance, and compute some basic statistics:

Mean distance = 29.794 ± 0.017 mi. (Actually computed from 20 rides)

So, the relative uncertainty is at the 0.06% level.