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?



Dave Atkinson [6083 posts] 1 year ago

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

Super Domestique [1596 posts] 1 year ago

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  3

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

craig749 [33 posts] 1 year ago

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?

SammyG [274 posts] 1 year ago

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.

Colin Peyresourde [1630 posts] 1 year ago

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…...

craig749 [33 posts] 1 year ago

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?