A blog by runners. For runners.

Data comparison of Garmin 305 and Garmin Forerunner 610

As a treat for my PR in the Wisconsin marathon 12 days ago I bought myself the Garmin Forerunner 610 to replace my trusty 5 year old Garmin Forerunner 305. I was looking forward to wearing a watch vs a brick, reduced heart rate spikes and more fun, not to mention the 305 “menu” button was starting to fail… However, my first run out on Tuesday was a little disappointing.
Prior to the race I knocked out 5 mile easy runs at a pace of between 8:10 and 8:30 with a heart under 142 beats per minute. Last Saturday (one week after the marathon) I went out for a 3 mile recovery run and came in at 7:51 for the 3.2 miles with an average heart rate of 142 – all seems to be going smoothly with my recovery. On Tuesday (10 days after the marathon) I donned my newly acquired 610 and hit the path for a 5 mile recovery run. Based on my recently discovered heart rate reserve I wanted to keep my heart rate under 142 and set a heart rate alert and configured the watch “pages” to show all manner of info but I was very interested in the lap and average %HRR stats.
50 yards down the road the alert started screaming at me. No stranger to heart rate spikes on the 305 I assumed it was something similar but the heart rate on the display wasn't in the 200s but a more reasonable 156. Huh. I carried on and gradually decreased my pace and the heart rate started to drop too. I switched to the lap screen I'd configured but both the average and lap heart rates showed a blank. I had entered my resting heart rate so for now I'd carry on without it.
5 miles later I came back to the house and based on my vibrating, beeping task master's instructions I had slowed down my pace for the whole run to 9:03 for a 139 average. Not terrible but certainly not where I expected to be based on past performance. No temperature variations between today and the past few runs bar a degree or two. It must be the watch. My Garmin 305 must have been failing and mislead me as to my actual heart rate. I showered and grumpily headed into work. Would I have to run this slow in all my future training easy runs? Had I been training to hard and stayed healthy out of sheer luck? Should I say heart rate training be damned and just run my ass off and not worry about the workout prescriptions?
Hours later I felt pretty good for a 5 mile morning and as the coffee started to kick in my scientific mind found a solution.
A tail of two heart beats?  Comparing the Garmin 305 to the 610
This morning (Thursday – marathon + 12) I ran another 5 mile run, the same course, same temperature, same time of day as Tuesday (and for arguments sake not far from Saturday's fast run either) but this time armed with both Garmins and a heart rate strap dedicated to each. I concluded that if they differed, I had a problem and would need more data. It hadn't occurred to me that they might agree…
For the sake of scientific disclosure the 305 is running the 2.90 firmware, the 610 is running 2.20.
Comparison of the same run measured by a Garmin Forerunner 305 and a Garmin Forerunner 610 simultaneously
The green shaded cells show where the Garmin 610 was at least 1% higher than the 305 for a value, the red where the 610 was at least 1% lower than the 305 for a value.
The charts below are the result of overlaying Garmin 610 data on top of the 305 chart from Garmin Connect. For the most part they concur with the exception of a large heart rate spike on the 305 at the start and an off the chart pace of 1:48 at the end of the timing chart.
Garmin Connect charts overlaid to illustrate differences between Garmin 305 and Garmin 610 over the same run simultaneously


The time difference vs. moving time is because 610 was also on auto pause. Despite stopping and starting both watches the 610 still auto paused and resumed after I restarted the timer when the traffic lights changed along the way, hence average pace was also off.
The overall moving time difference was negligible with the only difference probably due to not being able to start and stop each watch at the same time.
The average moving pace was also within my 1% limits – 1 second difference so no problems there.
The best pace for lap 5 on the 610 was a crazy 1:58 per mile spike not picked up by the 305 but the impact on average moving pace was too small to push the lap average in the wrong direction. The only big moving pace difference was lap 2 where 610 showed 8:52, 305 showed 8:45.
I was delighted to see my average heart rate on both devices didn't vary by more than 1% either way as I first thought it was at least 10 beats per minute off based on my first run.
The max HR was different by 9 beats per minute but I'm not surprised given the different profiles of the heart rate spike at the start of the run. The 610 appears to be better at handling HR spikes.
The calorie calculation seems very different at 648 on 610 vs 721 for the 305 despite identical weight data. I'm not too concerned though as I would expect the newer technology to be more precise.


The things I care about HR, average moving pace, distance, moving time on the 610 were all within 1% of the 305 so I'm happy that the watches are comparable in terms of data accuracy (relatively speaking). The differences that do exist don't impact my running. I also discovered that the reason the heart rate reserve didn't show on Tuesday was because even though I entered my resting heart rate I still had it calculate my heart rate zones by max heart rate. Why this would control whether or not to calculate heart rate reserve is beyond me but switching it to %HRR for zone calculations fixed it.

Don't shoot the messenger

So my theory about the 610 over-reporting my heart rate was proved to be false. It's me who is defective ;-) I'll need to run a follow-up experiment to prove this but a couple of new theories to test are:

  • Running in the morning vs. afternoon. I ran a lot of evening runs pre-marathon but the morning ones were still faster than this but I'm willing to test it out.
  • The addition of orange juice to my coffee and oatmeal breakfast before I run. I read in Pete Pfizinger's Advanced Marathoning that drinking orange juice with your morning oatmeal instead of coffee results in 3 times as much iron being absorbed by your body. I added the orange juice to my morning ritual with a delay on the coffee, but still drinking it (can't skip that part of my day). Could it be that my body is just too busy using heart rate cycles on my digestion?
  • The amount of time between finishing breakfast and running. Similarly to point two – I used to give myself about an hour between breakfast when I ate in the morning and running but with my new schedule with the twin girls I tend to sit in the nursery with them eating my breakfast and playing with them while my wife gets ready. I need to try to eat earlier before heading into the nursery to see if that makes a difference.

If all of these prove inconclusive it could just be that I am still recovering from the marathon 12 days ago and my body is communicating through my heart rate. I certainly feel fresher training at 70% of my heart rate reserve than I remember before the marathon on my recovery runs. It will be interesting to see if the rest of the hard works still get me where I want to be. I *feel* like I should be running faster but only the results will tell! I'll follow up with more of my experiment results.