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From Fellrnr.com, Running tips
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=Physical Characteristics=
The Fenix 5X is a big watch, and it's larger than I'm comfortable with as a 24 hour/day activity tracker. However, on the run it fells okay even on my diminutive wrists. The watch straps rotate where they connect to the watch body, unlike the Polar V800 where they extend from the watch body and therefore don't conform as well. The materials are excellent quality, and while it doesn't have the simplistic elegance of the [[Suunto Spartan Ultra]] it's still very nice. The exposed bolt heads make the Fenix 5 look rugged and sporty, though it's a bit fussy visually. The only thing that feels a little cheap and tacky is the watch strap itself on the cheapest version. It's nothing like the silky smooth and soft watch material that Suunto use.
=GPS Accuracy=
I've tested the [[GPS Accuracy]] of the Fenix 5X with its initial release firmware, and it's possible that its accuracy will improve as the firmware matures. (I tested using the 4.20 version of the GPS firmware, which is included in the 2.40 version of the overall firmware.) So far, the Fenix 5X certainly has 20 of room for improvement. At best, the GPS accuracy could be described as mediocre, and I think it's bad enough that it could significantly mess up your training. If you look at the middle, curved section, you can see that the Fenix 5X is doing particularly badly here. This is a challenging section, and reveals any weakness in a GPS watch. The right most segment with the right-angle turn is a section of the most watches do reasonably well on, but the Fenix 5X is struggling here as well. The tracks don't look too bad, you can see that the Fenix 5X is not able to calculate the distance correctly. The Fenix 5X doesn't get too badly confused going under the bridge, actually looks a little better in that area than the two Suunto watches shown for comparison. The blue lap markers are rather widely spread, again giving more evidence of poor GPS accuracy. It's possible that other versions of the Fenix 5 might have different GPS accuracy. It's possible that the plastic 935 might do a little better, but without testing it's impossible to know. (I buy all my test gear through retail channels. This allows me to be brutally honest in my reviews, as I don't need to keep the manufacturer happy in order to continue getting free samples or early access. The downside is that I'm more limited in the array of watches I can test. Therefore, it's unlikely that I will test other versions of the Fenix 5 at this point.)
{| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
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|[[File:BridgeFenix 5X.jpg|none|thumb|x300px| The GPS tracks from the Fenix 5X. This diagram has tracks color coded with green indicating good accuracy through to red indicating poor accuracy, and the lap markers as blue dots.]]
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|[[File:BridgeSuunto Spartan Ultra 1.6.14.jpg|none |thumb |x300px|For comparison, here are the tracks from the [[Suunto Spartan Ultra]], which had appalling GPS accuracy when it was first released, but the latest firmware has improved things.]]
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|[[File:BridgeSuunto Ambit3 R.jpg|none |thumb |x300px|Here's how the [[Suunto Ambit3]]'s accuracy looks.]]
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|}
As with all the watches I test, I first left the Fenix 5X in its "ready to go" mode with a clear view of the sky for over 30 minutes. This is to ensure that the chipset has ample time to download the almanac from the satellites, which should only take about 12 minutes. I also give the Fenix 5X at least five minutes after it has got a satellite lock before starting my run. This is to give it the best possible chance of having good accuracy. (Like most modern GPS watches, the Fenix 5X will download information on the satellite orbits when it's synced, but this predicted information is not as good as the live for data transmitted by the satellites themselves.)
=Optical Heart Rate Monitoring=
I've not found any Optical Heart Rate Monitoring (OHRM) implementation that's good enough to be useful, and the Fenix 5X is no exception. I believe that you're better off having no heart rate data than bad heart rate data. While a chest strap based heart rate monitor can have accuracy issues, these are generally dramatic and obvious, whereas OHRM can be quite misleading. (The issues with a chest strap heart rate monitor are also usually fairly easy to remediate, either with some electrode gel or a new battery.) The accuracy of [[Optical Heart Rate Monitoring]] (OHRM) will depend on a number of factors:
