I use Sense 2 and the accuracy isn't great but enough for recording purposes. The live pace on the watch is never accurate mostly all over the place so I don't use it. Distance - that depends. One thing which makes Fitbit not a great choice for runners is the lack of consistency. It is ok if the distance is slightly less or more as long as it stays consistent. In running, it's not always about absolute numbers. With Fitbit, I can't tell whether I perform better or worse because it is a lottery when it comes to metrics. Running the same route you won't get the same distance (tiny differences of a few meters are acceptable but I'm talking the difference of 100m-200m on a 5k distance - that's a lot). In other words, Fitbit isn't made for running.
There may be a few things that can improve that.
- Run on a relatively flat surface
- Wait for GPS to connect (if you use automatic stride, bad GPS may affect it for your next runs)
- You may control your stride length to keep it fixed (which I don't recommend as this may lead to poor technique and injury)
- You may use a different watch dedicated to runners
On my side, I kind of figured out why there is a problem (although there is no workaround, it's the design of run tracking). I run mostly with a fixed and high cadence (190spm+ with a lot more on downhills) which means that I control my pace by reducing or increasing my stride length (most advanced runners run with consistent cadence through the whole run, safer for joints and tendons, adds certain rhythm to the run). Fitbit however works better if the stride length oscillates around a fixed number which means you'd need to change cadence to maintain your stride length and keep pace up. This is particularly a problem on uphills and downhills where stride length may change drastically. The GPS doesn't seem to affect those metrics but the distance recorded by the GPS will affect your stride length for the next runs. Now, I don't know what Fitbit does with segments for which GPS lost connection but I noticed a drop in metrics accuracy in the next runs when previously GPS didn't function properly.
Fitbit distance measurement works quite well on treadmills because treadmills will force runners to keep stride length fixed (although the accuracy will depend on the incline and ideally, the runner should modify stride length with treadmill incline, these are small differences but will end up with big numbers when pace and distance change). This doesn't work in outdoors with a variety of inclines, types of terrain etc. There's not much else you can do. Fitbits are not sports watches. They do things very basic way and they are just meant to record your activities (lack of analytics tools shows that training with Fitbit is not part of the intended usage). If you are serious about running, just look for a proper runner's watch.
