Comparing the Garmin Forerunner 620 and 735XT Watches

After having used the FR620 since early May 2015, I felt the need for something new because there are new technologies available that the FR620 family does not support, namely ConnectIQ and the optical heart rate sensor. I compared the recent Garmin models FR235, 735XT, 935 and the Fenixes, and ended up purchasing the FR735XT which pretty much is what the 635 would have become if Garmin had ever marketed one (although the FR735XT is a cheaper Fenix model while the 935 is the 630 descendant – never mind.)

Reading fellrnr’s GPS watches reviews I was a bit hesitant about the FR735XT as far as GPS accuracy (although he didn’t specifically test this watch but the Fenixes which share the same basic technology) but having tested it for a couple of weeks now (including various 10k and HM races) I must say that is is pretty much on par with the FR620, within a few tenths of a percent. Not outstanding, but accurate enough for me to not make me think about a Stryd footpod (which costs another half fortune).

What I liked about the FR620:

  • WiFi. I totally liked how the watch began uploading a run as soon as I came back home and entered the coverage area of my WiFi network. Too bad Garmin couldn’t squeeze WiFi into the FR735XT housing – physically or economically. But they may have had to leave it to the 935 in order to justify the higher price.
  • The FR620 automatically deactivated auto-lap when doing custom trainings (e.g. intervals). With the FR735XT, I need to explicitly switch auto-lap off when doing custom training because if not, both auto-lap and the custom training will write laps into the .FIT file, which is confusing at best. I thus created a custom training of 40x 1 km on my 735 (40 is the max number of repetitions) for normal runs and keep auto-lap switched off.

What I like about the FR735XT:

  • much more stable Bluetooth connection. The 620 forgot the pairing with my smartphone (and thus, Garmin Connect Mobile) again and again, and I had to re-pair them each time I ran outside of my home turf. Annoying.
  • instance pace from a footpod (with recent firmware). The FR610 family could do this but Garmin sadly never added this feature to the FR620.
  • the optical HR sensor — although its measurement stability could be better. Every now and then, the measurement is stuck somewhere between 120 and 170 or even higher. Unclear what causes this but this is a problem that also affects the FR235 according to some Garmin forum posts. To cope with this, I usually wait until the measured value establishes before I start running, and if the reading goes up without me moving or is above 90, I simply reboot the watch. Normally it will work thereafter. For most normal runs, this is sufficient for me, and I don’t need an HRM belt – I don’t like the belts because they cause abrasions if I use them too often. For 10k’s and HM’s, I still use the belt, though. (1) (2)
  • ConnectIQ. Particularly for long races (HM and M), proper pacing is paramount, and GPS isn’t accurate enough to display a correct instant pace. I still own a Garmin footpod and tried to calibrate it using fellrnr’s calibration tool on the 400m race track but the calculated value seemed to be wrong most of the time. With ConnectIQ, I could install a very handy app named Peter’s Race Pacer which allows you to re-calibrate pace and distance on the fly during a race simply by pressing the lap button at each kilometer or mile mark (an idea I advertised to Garmin two years ago but which they never acknowledged). Definitely worth the 5€ for the registration! I use the app together with the Garmin footpod, and so I have the best of both worlds: instant (step-by-step) pace from the footpod and the distance marks’ accuracy. To be used with the normal run settings (i.e. without the 40x 1 km custom training).

The FR735XT is basically a triathlon watch – but I did not yet have an opportunity of checking any of the swimming and biking functions because I have been focusing entirely on prepping Frankfurt Marathon 2017 so far. I may do an occasional short tri after Frankfurt, weather permitting.


(1) I recommend ditching the Garmin belt and get a Polar belt instead (12-13€ on Amazon). They are softer than the Garmin belts, machine washable at 30°C, and appear also to be more durable. The Garmin HRM module happens to fit exactly on the metal knobs – you may have to cut 1-2 millimeters of the plastic collar around the knobs with a sharp knife, though.

(2) I found it pretty straightforward to convert my old HRMv1 module (which came with the FR620) to HRMv2 by just pairing it with the FR735XT. The watch will recognize the v1 module and perform a firmware upgrade to v2 on the fly. Apparently, the movement and acceleration sensors and the processor are identical – or at least close enough.

Comments and additions are appreciated! Have fun running!

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