Trail/Ultra Running GPS Review Roundup

We’ve made significant changes to our testing analysis to better distinguish the lap by lap performers under the various conditions as well as ensuring fairer overall comparisons between the models by balancing the test conditions.

This has meant old review figures aren’t compatible with new ones, though figures have changed a bit the rankings and overall reviews haven’t. We’ll be re-posting them all in the next wee while.

Our Reviews and (what they mean to) You

Look, there is really no such thing as ‘the best trail GPS watch’ for everyone all the time. What we are trying to assess is how well each model we test does against a bunch of parameters. We are explicit and open about this so you can figure out which model might suit your needs. Best for me probably isn’t best for you, hell, best for me depends on what kind of event I’m doing in what conditions and what socks I’ve got on.

To bypass the intro and head straight to the review round-up click here

We are pretty selective in what comes up for review. Mostly because we actually have to get hold of the watch in the first place (and convince the trail-running owner to swap it out for the formal testing period – ie. a couple of months minimum). Basically to make the cut, the marketing material has to suggest the model has the chops to trail run for 14hrs then someone we know takes a punt a buys one. We don’t take watches directly from manufacturers and are skeptical of reviews that do (even if they do give them back).

The required endurance and specifications means the watches reviewed are all going to be towards higher end of things ($$$), so they should perform accordingly.

Our reviews have a specific trail focus and we try to be as objective as we can (see our test methodologies). We don’t go in to the full set of features each watch has. The reviews don’t reflect a watch’s ability in pure road running, let alone cycling, swimming, or triathlons. There are other review sites on the web that give a more general overview – check them out if you need more. Check them out anyway, it’s never a good idea to rely on one source to form an opinion.

We are also not reviewing watches to boost sales. We are reviewing past models as well as current in order to give runners a sense of whether an update is indeed an upgrade for them (if you like your watch just be happy, ignore reviews and go for a run).

Given the kind of testing required to come to a conclusion on a model, this is likely 6 months or a year behind the release date. That’s not going to get a buzz on for you early adopters. There’s just no getting around the fact that the time required to come to a conclusion that is robust and generalizable to most trail runners is significant.

What’s worse is that our results may well be out of date by the time you read them. Frequent firmware and app updates are par for the course now and these can change things. What doesn’t change though is the fact that you should never by a GPS watch on a promise of future improvements. Statements like “Yeah, it’s a bit buggy now but a firmware update will fix that”, “or that feature is coming in Q4 of 2025” are simply alternate realities as of today.

Current State of GPS Trail Play

So far our review pool includes Garmin’s fēnix 2, fēnix 3, fēnix 5X, FR910XT, Suunto’s Spartan Ultra, Ambit3 Peak and Ambit2, Polar’s V800, Sony’s Xperia Z5c, and TomTom’s Adventurer. Though not all of these have been written up yet.

As is discussed in the testing write up and each review, GPS positional accuracy is only indirectly related to accuracy in distance and pace. Watch based filtering and accelerometers can improve distance and pace performance considerably, but filtering can only do so much with rubbish data.

Updated August 2018 – As well as adding some new models we’ve revived the old FR910XT and done some retesting. Results are suprising, as the FR910XT takes out the distance accuracy by a small margin over the Ambit3. Elevation wise the fēnix 5X and Ambit3 both battle for top honor.

Overall though the Ambit3 remains top (unmatched actually) in terms of its overall performance across distance, elevation, and endurance.

Distance and Altitude Accuracy

Distance accuracy as plotted here is how often each GPS gets within 1% of the true surveyed distance for each recorded lap across combined test conditions (ie. how often is it spot on?). The FR910XT retesting has resulted in it edging out the Ambit3 for first place with 29% and 28% respectively. The Adventurer is in last place by a margin with only 1%.


Looking at lap distance again by absolute accuracy, here we are measuring the average across all laps in absolute terms (so 5% long and 5% short would both score 95% here). The top two remain the FR910XT and Ambit3 (both 98%), and the bottom goes to the fēnix 3 (96%). The middle order is switched around too. Unlike the 1% test above the margins are very tight with only a couple of percent between the models. The big differences in accuracy open things up in the difficult GPS conditions testing.


Altitude accuracy in the next chart is how often the model gets within 10% of the outing climb. The fēnix 5X takes it out here at 81% with the Adventurer second on 69%. Well back in last place is the FR910XT at 27%.


And again for absolute climb accuracy by outing (how far off the true elevation climbed each is), the Ambit3 at 91% takes it from the fēnix 5X, with the FR910XT again bottom at 71%.



Trackpoint Density Cloud

Darker colours indicate higher trackpoint density, a perfect trackpoint cloud would be a dark red line overlaying the grey survey line. The width/location of the cloud indicates ‘how wrong’, whereas the colour ‘how often’. The trackpoint cloud depicted here are from all runs over a subset of the surveyed route with varying conditions. The bottom left tracks in each frame have limited (though not terrible) sky GPS conditions, the rest are relatively open sky.

As a group average they get the course pretty much spot on though the per model versions show a lot of variation. See the individual reviews for details of each model.


Garmin fēnix 5X (click me)

The fēnix 5X is an evolved, slightly more accurate and reliable fēnix 3. The shed-load of features has also been added to,  big additions being excellent basemap with auto-routing (ie navigation) capabilites and an optical HRM. The altimeter accuracy is among the best we’ve seen yet and distance accuracy in easy conditions is solid. For all outings it hits the +/-1% lap distance accuracy 21% of the time. Unfortunately it all comes unstuck in difficult GPS conditions, where distance accuracy is still pretty much bottom of the group, with battery life also suffering as the GPS struggles to cope. Even with UltraTrac on there’s no guarantee it’ll even make 20hrs if there are trees on course. The kicker being that the’ve dropped the wearable charging cable design from the fēnix 2/3. That said, if you are a fast 100km’er or short trail runner and are happy wearing a footpod there’s a lot to like.

Suunto Spartan Ultra (click me)

After +5000km on the wrist we’re still finding it hard to summarise the Spartan. Is it like a fēnix 3 with better accuracy but fewer features? Or an Ambit3 that’s less accurate with different and less useful running/outdoor features, but with a colour screen? Or something else again? It’s very accurate distance-wise in easy GPS conditions but pretty mediocre in more difficult conditions typical of NZ trail running (though it has improved with FW updates). For our combined outings measure it hits the +/-1% lap distance accuracy 22% of the time. It does have solid elevation climb accuracy. It also has an excellent screen, but rather limited in its screen customisation, and lacks a number of key trail/ultra running features. On the basis of current testing we couldn’t recommend it as an upgrade or a first purchase. The equation of accuracy + features = price just doesn’t stack up when compared to the other options available.

Garmin fēnix 3 (currently being rewritten)

The fēnix 3 has certainly come along way since our August 2015 pre-3.60 “avoid this model” review. If you want a watch to find your phone, skip your music list, and notify you of appointment, txts, etc, and can also do some trail GPS then this is the one for you. You will have to give a bit ground to most other models as the current fēnix 3 firmware version is credible though not stellar in accuracy or reliability. Hits the +/-1% lap distance accuracy 12% of the time. Ultra runners will also want to take note of the 12:30hr real world battery life and occasional UltraTrac reliability issues we’ve noted. Production quality also appears a bit hit and miss as two of three we’ve had ended in a warranty return within a month as it was recording complete nonsense. However, get a good one and the fēnix 3’s design and feature set is excellent Garmin are commendable on their openness of data exchange and fully featured Android app (even if the firmware/app upgrades break connectivity from time to time).

Suunto Ambit3 Peak (currently being rewritten)

Good for serious trail and ultra-runners though lacking fancy features compared to current models. Top of the class in terms of consistent accuracy, reliability, and endurance. Hits the +/-1% lap distance accuracy 28% of the time. It’s design focus appears clear, trim any feature that impedes accuracy or endurance. Vibrate still doesn’t make the feature list, and screen design means trail legibility is limited compared with other brands, buttons also a bit spongy and suffer a bit of lag (no good for 400m splits). At the end of the 15.5hr trail test it came out on 1st with 19% battery remaining. No real waypoint management so no good for you OSM trail mappers. And you’ll need to swap out your ANT+ sensors for BLE if you are upgrading from the Ambit2 or Garmin.

Suunto Ambit2 (currently being rewritten)

Overall pretty similar to the Ambit3 Peak, reliable plus in all-conditions. Accuracy is practically the same, though the sampling rate is down and recorded distance is a bit less. Hit’s the +/-1% lap distance accuracy even less at 7% of the time. At just under 14hrs, the Ambit2 also has less endurance (battery and memory) than the Ambit3 Peak, it lacks activity tracking stuff, and no mobile upload facility. Though as a bonus your anybrand ANT+ sensors will work with it.

Polar V800 (currently being rewritten)

Unfortunately is not quite a complete trail running watch. Though apparently very accurate in easy GPS conditions distance accuracy varies quite a bit in difficult conditions. Hits the +/-1% lap distance accuracy a mere 12% of the time on our mixed condition trail routes. Endurance proved solid as it went a full 13hrs in real world trail testing. While the V800 has some excellent running features, sadly is still a bit locked up in proprietary Polar land, doesn’t fully support sensor standards and data needs to be wrestled to get into other apps. Android app is also finally reaching maturity after initially lagging the iOS version. However, currently the mobile Flow app refuses to sync runs most of the time.

Garmin FR910XT (currently being rewritten)

Will do the core business for trail running (most of the time). The granddaddies of trail run GPS watches, on most days it is still capable of showing the youngin’s a thing or two about accuracy. Hits the +/-1% lap distance accuracy 29% of the time. Third party developers also come to rescue with Android mobile upload capability. Unfortunately is also prone to occasionally overestimating things, and on a really bad day can get a bit confused about where it actually is (ie. is completely off-track). Elevation and climbing data is also a bit of a lottery and well behind current GPS models

Garmin fēnix 2 (currently being rewritten)

A lemon for trail runners, don’t even think about it. Prone to random reboots, eats your data before it’ll upload it. Battery life is very poor and UltraTrac endurance mode doesn’t work in trail conditions, and even if it did wouldn’t record your heart rate anyway. Positional accuracy highly variable. Suprisingly overall distance accuracy isn’t too bad (though it consistently runs a little long), but doesn’t save it from it’s ‘avoid at all costs’ trail rating.