I can do a ride with my wife, where we ride side by side on bike with literally identical tires-- 700x25 Giant Gavias. We both have speed sensors calibrated to the same rollout, because... it's the same tire.

Without fail, the auto-lap chime goes off for her at 5 miles, and I will be at 4.94. When hers chimes for 10, I will be at 9.88. And so on and so on, her always recording an additional 0.06 miles every 5 miles. Of course, this is 0.012 miles, or a "bonus" 63.36 feet per mile traveled.

This falls soundly into margin of error, even though it is repeatable. Best you can do is to get your computer to record as close to a mile as possible, and go from there. Or just throw caution to the wind and let it auto-calibrate. Might work in your favor, might not.

I have one health app that records exercise activity in the background, using GPS only, and auto-starting/stopping itself presumably based on velocity. It is generally about 5-7% short on distance.