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910XT GPS and Speed Discrepencies
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Has anyone else experienced something similar to this. Just for a comparison I had my 910XT and my 810 on my bike during my ride today. The 810 recorded my distance at 36 miles with a top speed of 27.9mph, the 910XT recorded my distance at 58 miles with a top speed of 79.5 mph. The 810 has the accurate information. Both of them are hooked up to the same sensors, Vector Powermeter, and the new version of Garmin's Cadence and Speed sensors.

I realize no two gps units will record the exact same distance, but a 30 mile and just over a 50mph discrepancies? Both of them have the latest version of the software from Garmin, and when looking at the GPS plot on Connect they match up. Looking at the other numbers, the HRM information is nearly identical, as is the Powermeter data, and the Cadence data. I'm assuming the speed sensor is the problem when it comes to the 910XT, but that's just strange to me.
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Re: 910XT GPS and Speed Discrepencies [mleech77] [ In reply to ]
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Post links to the workouts.
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Re: 910XT GPS and Speed Discrepencies [mleech77] [ In reply to ]
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Do you have a speed/cadence sensor? If so, did both of the units sync to it?
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Re: 910XT GPS and Speed Discrepencies [dfroelich] [ In reply to ]
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dfroelich wrote:
Post links to the workouts.

GC Links:
910XT = http://connect.garmin.com/...n/activity/589568247
810 = http://connect.garmin.com/...n/activity/589565207

JoeO wrote:
Do you have a speed/cadence sensor? If so, did both of the units sync to it?
Yes, I have the new separate garmin speed and cadence sensor, and both units were sync'd to them.
Last edited by: mleech77: Sep 14, 14 3:47
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Re: 910XT GPS and Speed Discrepencies [mleech77] [ In reply to ]
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If that is the case was your wheel size the same and correct for both?



Joel
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Re: 910XT GPS and Speed Discrepencies [mleech77] [ In reply to ]
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Yeah, it looks like there is a weird decoding of the speed sensor's signal in the 910. Those speed spikes are jumping to ~70mph (~35m/s) and do not correspond with any weird 'distance traveled' numbers. You can therefor assume that that the 910 is recording all your lat/longs correctly (and since the map looks fine).
I'd guess your 910 may be in need of service. First, try deleting all the workouts on it. I wonder if all those inputs (HR, power, speed, cadence) are giving it data faster than it can write, and so it spazzes out a bit. This may be exasperated by having less room to write to (which can slow it down more). Also, since those speed spikes are happening later into the ride, your watch will be even more full then.

...I dunno, its worth a try.

If your watch is under warranty, you can also use the lack of elevation data as a claim for replacement.

Good luck.
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Re: 910XT GPS and Speed Discrepencies [joelt] [ In reply to ]
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joelt wrote:
If that is the case was your wheel size the same and correct for both?l

Looks like this may be a potential culprit. The 810 was set to automatic and is 2081mm, the 910xt was set to manual and had no entry. When I set it to automatic, it is now 2100mm.

dfroelich wrote:
First, try deleting all the workouts on it.
Good idea. I was planning on deleting everything on it prior to my HIM this coming weekend just to be on the safe side. I will probably actually do a hard reset and see if that clears the elevation issue as well.
Last edited by: mleech77: Sep 14, 14 11:44
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Re: 910XT GPS and Speed Discrepencies [mleech77] [ In reply to ]
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mleech77 wrote:
joelt wrote:
If that is the case was your wheel size the same and correct for both?l

Looks like this may be a potential culprit. The 810 was set to automatic and is 2081mm, the 910xt was set to manual and had no entry. When I set it to automatic, it is now 2100mm.

Cool, perhaps your 910 was using GPS for the speed reading. Whenever that blanks out...it tried to resort to the speed sensor. Without a value it basically tried to divide by zero...and got a big number, but knocked it down to 70mph.

I bet that'll do the trick! Even so, its good to dump the old workouts often.
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