Read my earlier post. If you want reliably accurate data, but a $15.00 cycle computer with a wheel sensor. These can be calibrated to small fractions of 1%, and are extremely reliable and consistent.

That said, a 30% variance for GPS distance is way beyond the pale. Something else must be going on.

However, the nature of GPS errors is such that they tend to average out over time, so you'll generally get much more reliable data on longer rides than shorter. It's like tossing a coin, you might get 7 or 8 heads out of 10 tosses, but you'll rarely be much above or below 50 out of 100.