IDK ... my Samsung S4's GPS reports a slightly different distance on different runs - +/-2% - and generally the phone reports less distance than my computer (Cateye Solar), often by as much as 8%.

The Cateye reports about the same distance as Google Maps does. The Cateye, without fastidious setup, is more accurate than my phone.

Why is this important? I have some mileage goals that will be tough to reach this Summer, and a device that reports distances inaccurately can keep me from reaching it.

Sure, I know reaching my goals isn't a big deal in the scheme of things, and 8% is even less meaningful, but it's easy to get more accuracy than my phone provides, so I just keep my phone in my pocket with GPS turned off and use my 'puter.