I use Ride Report on an iPhone 6s. Mostly so I can the Health app show my mileage. Just gives distance and average speed, if memory serves. The results are not too far off a Polar v650, good enough for me anyway. It does not appear to drain the battery any more than the rest of the apps, and is quite good at automatically starting and stopping, although it does run in background 24/7. It is better as discerning car and bus travel than the first few weeks of use. Nothing like seeing my average speed of 50+mph! You can manually change the results.