From someone's review:
"When running indoors without GPS, the Forerunner 235 is able to use the in built accelerometer to estimate the distance run. This feature will become more accurate overtime after a few GPS activated runs outdoors. From my experience, the Forerunner 235 was able to estimate my treadmill run to 95% accuracy after multiple outdoor GPS enabled runs."
I definitely noticed errors above 5% when I first started using it in the field house, but it improved over time.