How accurate is Strava for the distance

Distance differences between the Strava and the cycle computer

There's a combination of two things that could be going on here. 1- The GPS track you have has some uncertainty (+ - 1 to 10 m, depending on the signal quality). Depending on the sampling frequency, it may appear like you're working in a zigzag, adding a little bit of space to your actual path. Some algorithms can smooth this out, but it's not 100% accurate.

2- The display of your bike computer may be based on the speed sensor which uses the rotation of your bike and the diameter of that bike. It multiplies the number of revolutions by the circumference of the tire to get the total distance traveled. There is a setting in your bike computer with which you can change the tire circumference and make it more precise. The scope depends on your wheel size, tire size, air pressure and wear. If it is not accurate, you will receive an incorrect distance measurement: https://www.cateye.com/data/resources/Tire_size_chart_ENG.pdf lists some of the circumferences for different wheel and tire sizes

Rider_X

With respect to (1), if the error is random, the measured average distance should be an unbiased estimate of the true distance. (i.e. a single trip may indicate another, but if you average all of your individual trips your average should converge on actual distance as your sample size approaches infinity). The OP saw a consistent and sustained negative bias in the measured distance for the same route, suggesting that a random measurement error is not the source.

Jean-Michel Roy

Agreed, I read "recently bought" and assumed it wasn't replicated many times. If the sample size is large enough, you are right that this is probably not the problem.