Garmin speed/ascent help?

by craig749   May 15, 2014  

The last few days of riding my bike I've noticed that the Garmin 510 has been dropping speed. For example I'll be doing 18mph then it will drop to 8mph then go back to 18 then go to 6 etc etc. then once I finish and save the ride it says new ascent record 22,000 feet???

Has anyone experienced this?


6 user comments

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are you using a speed/cadence sensor or just the GPS?

Dave Atkinson's picture

posted by Dave Atkinson [7830 posts]
15th May 2014 - 21:40

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Occasionally my Edge 200 does this. Usually in the same spot, heavily covered by trees. Generally I'm sitting about 28-30kph at this part of this particular route but it will register a drop to about 24.

That said, today it rocketed up to 46 at the same spot so it ain't all bad Wink

But basically, yes, mine does it and it's annoying.

posted by Super Domestique [1686 posts]
15th May 2014 - 22:01

1 Like

Yes I used a speed/cadence sensor. It is filthy at the moment so perhaps the speed issue may improve once it's cleaned. The thing I find most strange is the rediculously high ascent readings. Maybe a factory reset might help?


posted by craig749 [61 posts]
15th May 2014 - 22:18

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The barometric altimeter is bad even when working properly, when analysing rides I check the use map data button on garmin connect to bypass the bad data from the unit.

Hard reset:

Power device off
Press and hold Page/Menu, Start/Stop and Power simultaneously
Release buttons when the word "Garmin" disappears

910xt fix:

To determine if your barometer is faulty. go in to the diagnostics mode:

1. Power off the 910XT

2. While first pressing and holding down the Enter button, press and hold the Power button.

3. Continue to hold both the Enter key and Power button until the diagnostic screen appears.

4. Release both buttons.

Now that you are in the diagnostic screen, press the Mode button, exactly 7 times. The 910XT should beep each time the Mode button is pressed.

If the third line called “Baro Alt” shows 65,000 ft or 20.00 m and the fourth line under “Baro Pressure” shows 0, then your unit is faulty.

Power off the 910XT to exit out of diagnostics mode.

Call Garmin at 1-800-800-1020 with this information provided and they will send you a new watch.

If you are out of the warranty period, as I was, they will send you a factory refurbished unit, even if the warranty period has expired.

posted by SammyG [295 posts]
16th May 2014 - 13:04


I think what Dave is suggesting (at the risk of putting words in his mouth) is that without the cadence/speed sensor the GPS is not always as good as it should be when it comes to recording speed, and altitude.

I switch between my road (training) and mountain bikes (commute), but haven't got round to getting a spare sensor for the mountain bike, and get similar readings to you. I know I'm doing 16-18mph, but it reads 9-12.

Sounds like you need to clean your sensor, or, for want of sounding facile change the battery, or check your magnet is intact…...

posted by Colin Peyresourde [1574 posts]
16th May 2014 - 20:25


Thanks for the detailed advice......I've given the sensor/magnet a good clean so hopefully it will start working as normal. When I look under System>GPS on the unit it shows GPS Mode as being off and Set Elevation as -oo43? any ideas what these should be set to?



posted by craig749 [61 posts]
17th May 2014 - 21:24

1 Like