# New Garmin 810 - Distance WAY off!!



## bikesordeath (Sep 17, 2011)

I recently purchased a Garmin 810 and the distances are always much lower than everyone I ride with. As an example here are some numbers from todays ride with my gf. We road together the entire time, so our numbers should be pretty close.

This is the bike computer on my go's bike:








This is a screenshot of her Strava captured using iPhone 6








This is my Apple Watch capturing data from my iPhone 6








This is my Strava data captured using Garmin 810








Sorry they are sideways. To make it easier to read:

GF's bike computer: 17.69
GF's iphone 6: 16.7
My iPhone 6: 16.11
My Garmin 810: 14.4

I've been playing with all the settings on the 810 trying to figure it out with no luck. Am I missing something or is my device defective?


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## bikesordeath (Sep 17, 2011)

ttt


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## Daflake (Nov 2, 2015)

First make sure you are up to date with firmware. Secondly check your wheel size in your bike profile. I usually leave mine to automatic, but if you have set it, it might be incorrect and causing the discrepancy.


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## honkonbobo (Nov 18, 2006)

are you using a wheel sensor or GPS only? if its the latter, put on the wheel sensor and you will likely improve your accuracy.


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## my40thz (Nov 6, 2015)

I would say signal loss classic garmin issue.


Sent from my iPad using Tapatalk


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## Harold (Dec 23, 2003)

What is the sampling interval?


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## upstateSC-rider (Dec 25, 2003)

Stop taking shorcuts.


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## bikesordeath (Sep 17, 2011)

Daflake said:


> First make sure you are up to date with firmware. Secondly check your wheel size in your bike profile. I usually leave mine to automatic, but if you have set it, it might be incorrect and causing the discrepancy.


Firmware is up-to-date. Mine is set to automatic as Garmin doesn't have a code for my wheel size.



honkonbobo said:


> are you using a wheel sensor or GPS only? if its the latter, put on the wheel sensor and you will likely improve your accuracy.


I've tried both



Harold said:


> What is the sampling interval?


Huh? How long have I had the issue?


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## bikesordeath (Sep 17, 2011)

my40thz said:


> I would say signal loss classic garmin issue.


That's disappointing if a phone has better gps signal than my gps device.


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## Daflake (Nov 2, 2015)

I would have to agree with you there. I have an 810 and have not had the problems your are seeing, so something isn't right. I just took a look at mine and can't see anything that would affect this. Might be worth giving Garmin a call on it and see what they say.


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## bikesordeath (Sep 17, 2011)

Daflake said:


> I would have to agree with you there. I have an 810 and have not had the problems your are seeing, so something isn't right. I just took a look at mine and can't see anything that would affect this. Might be worth giving Garmin a call on it and see what they say.


Thanks for letting me know. I contact my LBS where I purchased it. They are going to let me borrow one and see if I get different results.


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## Nutball (Oct 11, 2008)

I doubt there is anything physically wrong with your Garmin.

First off, I highly recommend using a wheel sensor as relying on the GPS for distance will always be problematic unless your riding on the road. Even with perfect satellite coverage, GPS only measurements will tend to be read short. This is a byproduct of the relatively slow sampling frequencies of consumer GPS receivers (1-sec in this case). This isn't much of a problem on the road, but the slow sampling frequency has the tendency to clip sharp corners off the trail. 

Second, I don't ever recommend using automatic for the wheel size. It has never worked out for me for either of my Garmins. I also don't recommend using one of the presets as they generally aren't particularly accurate either (can't be since they don't account for tire pressure, rider weight, labeled size inaccuracies, etc.).

Enter the size manually after doing a roll out using your preferred tire pressure WITH your weight on the bike. Google how to do a roll out to calibrate your bike computer if you haven't heard the term.

I also strongly prefer disabling SMART recording. Use the 1-second recording instead. In theory, it shouldn't change the distance much but your tracks will look much better.

Good luck,
Nutball


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## Harold (Dec 23, 2003)

Do not assume the interval is 1 sec. If the OP did not change it to 1sec, it is probably on "smart" recording as default. Which if the OP had done some reading, he would see that that setting is terrible for the mtb on twisty trails.


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## bikesordeath (Sep 17, 2011)

Thank you! I've changed it to 1 sec. I'll give it a go and update the thread with my results.


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## Nutball (Oct 11, 2008)

Again, I also recommend switching to manual wheel size for mountain biking (I am assuming your mtn biking from your trace image). In my experience, auto works well for a road bike. However, when I tested it on my mountain bikes the automatically calibrated wheel size would vary 10-15% from ride to ride with no changes in tires, pressures, etc. If I happened to be in an area with spotty sat coverage when its algorithms decided to recalibrate my wheel size - I would get an inaccurate measurement. If I happened to be on a section of trail with lots of turns when it decided to recalibrate - I would get an inaccurate measurement. In my opinion, it is better to be a bit high or low with a manual wheel size entry because that error is at least consistent.

If you don't have a tape measure to do a roll out calibration then I recommend taking an automatically calculated wheel size (from a road ride) and locking that measurement in manually. After a ride you can always tweak your manual entry up or down a little if you know its off. For example, if you consistently read 10% lower than everyone else in a group ride - you could tweak your wheel size entry up by 10%. 

GPS distance data is not really that accurate for a sport like mountain biking where you are rarely moving in a straight line. Confession: my local trails have lots of tight turns and lots of tree cover so my situation is a tough case for GPS. Second confession: I use one head unit for several bikes and I think the Garmin recalculates every time I change profiles (when set to automatic) which exaggerated my issues. Third confession: I have integrated GPS receivers into a few devices for my day job as well - GPS inaccuracy isn't just a biking issue.

The biggest downside to manual calibration is that you need to remember to change the setting when you change tires.

my two cents


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## rockhop (Mar 22, 2006)

As per Garmin support from their forums:

_The GPS sampling rate on the Edge 500 and all other fitness products is once per second. The sampling rate is not effected by the recording rate which is usually variable (smart recording). There is no way to adjust the sampling rate on any of our products._

As expensive as some of these GPS units can be, they're still just toys. And unless you don't want to spend well over $1k you'll just have to accept the margin of error. Your Garmin simply won't track distance as accurately as a professional GPS receiver. However a $35 old school CatEye properly set up will have less margin of error!

For better info as to what I mean, http://www.esri.com/news/arcuser/0104/rec-gps.html


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## my40thz (Nov 6, 2015)

I use mapmyride app and my pebble watch. Total cost free app 100.00 dollar multipurpose watch.









If I ever find myself lost, I just change where I'm going.


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## ghettocruiser (Jun 21, 2008)

Wait, your Garmins are auto-recalculating the wheel size for_ every ride_?

I know the 500 just did it once for a newly detected wheel sensor and saved it until the user instructed otherwise.

I haven't seen the 520 _claim_ to be recalculating a previously auto-calculated wheel size, yet. I have't monkeyed with it beyond that.


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## Harold (Dec 23, 2003)

rockhop said:


> As per Garmin support from their forums:
> 
> _The GPS sampling rate on the Edge 500 and all other fitness products is once per second. The sampling rate is not effected by the recording rate which is usually variable (smart recording). There is no way to adjust the sampling rate on any of our products._
> 
> ...


Garmin is just using different language to explain the same thing. OP did not show us distance as reported on the trip screen of the Garmin. He showed us a screenshot from Strava, which is reporting distances calculated from the RECORDED data. For our purposes here, it doesn't matter what we call this interval. As you said, we cannot change how often the device samples from the satellites. But we CAN change how frequently it records the positions into its tracklog. The OP made that change, so it will be better than before. A wheel sensor will improve things even more, too.


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## WR304 (Jul 9, 2004)

down2ride said:


> To make it easier to read:
> 
> GF's bike computer: 17.69
> GF's iphone 6: 16.7
> ...


There are a few settings to look at to try and sort this out.

On the Garmin Edge 810 the recording interval should be set to 1 second recording (not smart recording)
On the Garmin Edge 810 the Auto Pause should be set to OFF
On the Garmin Edge 810 with speed sensor the wheel circumference should be set to Custom ( not automatic) and you should use a measured compressed circumference for your tyre (not a number from a table or the circumference from a roll out test without a rider on board).

If you're losing a lot of distance this is most likely to be down to a combination of the settings above. When mountain biking at lower speeds Auto Pause (where the timer automatically recognises if you are stopped or not and decides whether to start or stop the timer) can cause problems. It can activate in error, cutting off parts of a ride where you were actually moving, which would see you lose distance. Leaving the timer running throughout the ride, and then letting Strava sort out stationary and moving time when you upload the ride afterwards, is usually the best option.

On your girlfriend's Cateye bike computer the distance travelled relies on the wheel circumference entered. As with the Garmin Edge 810 you should use a measured compressed circumference for the tyre on the wheel with the speed sensor attached (not a number from a table or the circumference from a roll out test without a rider on board).

My method for measuring tyre circumference:

Wheel Size - CUSTOM and use a manual tyre circumference so that the GPS isn't involved. Whenever you fit different tyres this needs changing. Don't leave it on auto.

Because an offroad tyre at low pressure compresses so much when you sit on the bike if you just do a measured wheel circumference roll out with no weight on the bike this is going to be inaccurate (too large a circumference). I typically have a few favourite tyre types that I use most of the time and use a track pump so they always start off at the same pressure for consistency.

For each tyre at the air pressure that I intend to use I measure the moving wheel circumference whilst actually riding by putting a small blob of white grease on the centre of the tyre tread (something that is easy to identify, will leave a mark and show up on the ground) and then ride along on the bike for a few metres sitting in the saddle as I would normally. The front and rear tyres compress different amounts so you need to measure the one that your wheel magnet and sensor will be attached to.

Using a tape measure you then go back and measure the distance between two of the white blobs left on the ground as the wheel rotated. That distance is your actual moving wheel circumference to enter into the computer settings for that particular tyre.

That ought to give you distance measurements that are closer to the others.


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## Harold (Dec 23, 2003)

WR304 said:


> There are a few settings to look at to try and sort this out.
> 
> On the Garmin Edge 810 the recording interval should be set to 1 second recording (not smart recording)
> On the Garmin Edge 810 the Auto Pause should be set to OFF
> ...


Ah, yes, I forgot about auto pause. That's another important one on Garmins. I disable auto pause and let software sort that out. But even still, Garmin and Strava and everybody else use slightly different cutoffs to determine stopped vs. moving time, and that will always affect final measurements.

I don't know about the Edge 810 specifically, but I know that on some models (like my Forerunner 310XT), you have to let the computer auto-detect wheel diameter before it will let you enter a custom number. I haven't bothered to modify that custom number on mine because I modify my tire pressures on my fatbike quite a bit depending on conditions. Especially in the wintertime. I have been satisfied with the auto method so far, but I haven't used the wheel sensor through the winter yet. One thing I do know is that I'm not going to do a rollout every time the snow conditions change and I adjust my tire pressure (which can be daily here).


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## WR304 (Jul 9, 2004)

Harold said:


> I don't know about the Edge 810 specifically, but I know that on some models (like my Forerunner 310XT), you have to let the computer auto-detect wheel diameter before it will let you enter a custom number. I haven't bothered to modify that custom number on mine because I modify my tire pressures on my fatbike quite a bit depending on conditions. Especially in the wintertime. I have been satisfied with the auto method so far, but I haven't used the wheel sensor through the winter yet. One thing I do know is that I'm not going to do a rollout every time the snow conditions change and I adjust my tire pressure (which can be daily here).


On the Garmin Edge 520 and Edge 500 you can set a manual circumference as soon as the sensor is paired. It doesn't need to auto detect wheel diameter first.

I just paired up one of the new Garmin magnetless speed sensors and as soon as it was paired (indoors no GPS lock) the option to manually set the circumference appeared on both Garmins.

What I did was to make a spreadsheet listing the compressed tyre circumference at each of the common pressures that I use for the front tyre (20psi offroad, 40 psi onroad being the usual ones.) I can quickly lookup the tyre circumference and change it in the Garmin as needed to match the tyre pressure that I'm using on that day.


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## Harold (Dec 23, 2003)

WR304 said:


> What I did was to make a spreadsheet listing the compressed tyre circumference at each of the common pressures that I use for the front tyre (20psi offroad, 40 psi onroad being the usual ones.) I can quickly lookup the tyre circumference and change it in the Garmin as needed to match the tyre pressure that I'm using on that day.


That's a good idea. I can just do the rollouts for each pressure once, then. Most of the summer, I'm running about 9psi give or take (I definitely won't change the measurement for a .5 psi adjustment I might make out on the trail), but for deep, fresh snow, I might go as low as 2psi. It doesn't sound like a lot of change in pressure, but it's absolutely massive. 9psi is pretty firm, and I'm willing to tolerate riding a little bit of pavement at that pressure to connect trail segments and such (or do urban rides with stairs and drops and stuff). 2psi is nearly down onto the rim when compressed with my weight. If I'm doing a pavement-specific ride that's not an urban ride with stairs and stuff, I'm riding a different bike.


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## Daflake (Nov 2, 2015)

All good ideas, but I can say that I don't do any of them and my Garmin 810 is fairly accurate with distance. The only thing that I have done is to set auto pause to only when stopped. I ride road and mountain with no problems with distance.


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## Harold (Dec 23, 2003)

Daflake said:


> All good ideas, but I can say that I don't do any of them and my Garmin 810 is fairly accurate with distance. The only thing that I have done is to set auto pause to only when stopped. I ride road and mountain with no problems with distance.


I see you live in AZ. OP lives in Texas, and I live in Indiana. Both are states where there's not a ton of public land. As such, trail networks tend to be more dense. Especially here in IN, where the terrain tends to be pretty rolling in some areas and trails can be packed into small areas quite densely. This makes the trails extremely twisty. I suspect parts of TX are much the same way, though I only have experience with trails in a relatively small part of the state. There are some trails where the wheelbase of my bike strongly correlates to how quickly I can take some of those corners. I've lived out west (UT), so I have a reasonable idea how much straighter trails tend to be. There's a ton more land, so trails don't HAVE to be packed into small areas. Trails can actually go from point to point, rather than looping all over making you feel like you're on a hamster wheel at times.

Trail layout matters a lot when we're talking about the accuracy of recorded GPS data. The more twists and turns in a trail, the shorter a GPS-only record of the ride will be. Where I live, GPS-only data can shortcut a trail by almost 2 miles in 14 quite easily (an example from a local trail). Dense forest cover also means that when you're "stopped", auto pause won't be able to tell because the position is wandering at a "speed" above the GPS's cutoff for determining what "stopped" really is. Or, it has to be set by speed, and it thinks you're stopped when you're actually pushing your bike up a steep fire road. Best not to use it at all.


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