Originally Posted by JohnJ80

Depends on a couple of things:

1. How accurately you set the tire circumference in the bike computer. Most people get this wrong and that leads to significant error.

2. How long the ride is.

GPS, when in good coverage, can be accurate to within 13 feet. So if you did a long ride, your start would be known within 13' and so would your finish. If the error was at max worst case, that would then be 26'. If you did a 100' ride, that would be horrible accuracy. If you did a ride of 10 miles, it would insignificant (<0.05%).

Note that changing tires sizes can have a big impact on wheel circumference and distance accuracy.

GPS based Bike computers typically compute the wheel circumference over many wheel revolutions. I would guess they update that on occasion. Cycling computers without GPS, you have to set it and there is no check on that over time.

FWIW, I found I had entered my wheel circumference incorrectly when I compared the results given by my GPS based bike computer. So I'd go for the bike computer as accurate (or the gps in your phone) over distance.

J.