Garmin 705 inaccurate mileage?

tomo33

I completed the 50 mile Sportive in Abergaveney on Saturday.

Started the 705 exactly as i started.

I was concerened when other riders said we had circa 10 miles to go and the Garmin indicated more circa 20% more (It was a relief on this occasion as it was the first 50 mile cycle ride i had completed,it was very hot and the route i found very hard)

On serious note i stopped the 705 as i passed the finish line and the reading was 42 miles. I have a speed and cadence sensor and have set the bike profile to auto as per the instructions.

Any advice on setting and potential problem would be helpful

Regards Steve

redddraggon

My Garmin 705 is pretty much bang on. No noticeable difference to my magnet+sensor computer.

jibberjim

tomo33 wrote:

On serious note i stopped the 705 as i passed the finish line and the reading was 42 miles. I have a speed and cadence sensor and have set the bike profile to auto as per the instructions.

Did you see it pop up and say "Wheel size computed" ?

Until that happens the distance is potentially inaccurate, have a look at the log in SportTracks, see if it's accurate 50miles and where you went.

amaferanga

If you're using a speed/cadence sensor you should set the wheel size manually. The auto-calculate wheel size on Garmins is notorious for getting the calibration wrong.

Oh and a sportive isn't a race

plowmar

Too true amaferanga always set up the wheel circs. on any computer never leave it to, what could be, chance.

There will be a table in the computer instructions to input the wheel dimensions your self tomo33

But don't get hung up just enjoy yourself and change the mileage in the bike profile section.

Karl2010

Input the wheel size manualy mate.
You need to do a "roll out".

Get a marker pen and draw a line away from your rim on the 'tyre wall'.
Set this mark so its at the bottom of the wheel/on the floor.
Then draw a mark on the floor that lines up with the one on the wall of your tyre.
Sit on the bike and roll forward one rotation of the wheel. (use a wall or another person to steady you)
Once you've rolled forward one rotation, put another mark on the floor that lines up with the one on the wall of your tyre.
Put the bike to one side.
Get a tape measure and measure between the two marks on the floor, and that is the circumfrence of your wheel. Thats the number you manualy input into your garmin.

When you do the roll out, go in a stright line to get an acurate measurement.
By sitting on the bike when you do the roll out you compress your tyres, which makes the circumfrence of your wheel smaller.

hope that helps

ravey1981

Karl2010 wrote:

Input the wheel size manualy mate.
You need to do a "roll out".

Get a marker pen and draw a line away from your rim on the 'tyre wall'.
Set this mark so its at the bottom of the wheel/on the floor.
Then draw a mark on the floor that lines up with the one on the wall of your tyre.
Sit on the bike and roll forward one rotation of the wheel. (use a wall or another person to steady you)
Once you've rolled forward one rotation, put another mark on the floor that lines up with the one on the wall of your tyre.
Put the bike to one side.
Get a tape measure and measure between the two marks on the floor, and that is the circumfrence of your wheel. Thats the number you manualy input into your garmin.

When you do the roll out, go in a stright line to get an acurate measurement.
By sitting on the bike when you do the roll out you compress your tyres, which makes the circumfrence of your wheel smaller.

hope that helps

Or use the valve as a marker....

blorg

The Garmin is different from normal computers; there is no need to do a roll-out as it only refers to the rear wheel speed sensor at all is when you are indoors with no satellite fix, e.g. on a turbo trainer. Outdoors it takes speed from the GPS. (At least this is my experience.)

@OP- many events misquote distance and so it is possible that it was only 42 miles. However if everyone else with a computer reckoned it was actually 50, I imagine your Garmin threw a wobbler. Best check the route in SportTracks as suggested and see if it is all there/what distance it reports.

Airwave

Have you got the auto puase set to high.i.e 10mph.You could lose miles that way i think?

Cleat Eastwood

blorg wrote:

The Garmin is different from normal computers; there is no need to do a roll-out as it only refers to the rear wheel speed sensor at all is when you are indoors with no satellite fix, e.g. on a turbo trainer. Outdoors it takes speed from the GPS. (At least this is my experience.)

@OP- many events misquote distance and so it is possible that it was only 42 miles. However if everyone else with a computer reckoned it was actually 50, I imagine your Garmin threw a wobbler. Best check the route in SportTracks as suggested and see if it is all there/what distance it reports.

I think you can do it either way from 305 0nwards, that is manually input the data from the chart in the manual or leave it, like you say, and ave speed recorded from gps.

Only the 205's dont have manual input, i think its something todo with the other garmins having a cadence sensing facility.

blorg

Cleat Eastwood wrote:

I think you can do it either way from 305 0nwards, that is manually input the data from the chart in the manual or leave it, like you say, and ave speed recorded from gps.

Only the 205's dont have manual input, i think its something todo with the other garmins having a cadence sensing facility.

Yes, you can manually set wheel size, my point is AFAIK, it will ignore the sensor entirely as long as it has satellite reception.

jibberjim

blorg wrote:

Yes, you can manually set wheel size, my point is AFAIK, it will ignore the sensor entirely as long as it has satellite reception.

This is wrong, if there's a speed sensor, it takes the speed from that before satellite, as it's a lot more accurate - assuming the wheel size is correct. The distance track is also recorded from the speed sensor. However the points are the genuine GPS points (unless you have lock on road set when the GPS points will be normalised to the nearest road) SportTracks can read both the distance track and the GPS points.

blorg

That is not my experience with the thing; speed will stick for example in long tunnels or carparks.

amaferanga

I'm not sure about the 705, but I do know that Garmin are not consistent on this through their range. Its quite possible that the Edge 500 does something different from the 705. Things also change between firmware releases. I have a Forerunner 310xt that I use on the bike with a GSC-10 and with the previous firmware release the speed/distance was always taken from the sensor. With the latest firmware release the speed/distance is always from GPS unless GPS is disabled.

Mike400

Slight thread Hi-jack

Ive an edge 305, and the elevation data is next to useless.

I can do a loop and when I return to the point I started from according to the edge Im 100ft lower......

When I ride into work it records the run as downhill all the way (correct) but on the way home it also records it as downhill even though there should be an overall gain.

Weirdly big hills show in the right place on the graph, but the starting altitude is always waay higher than the finish altitude regardless of where I cycle which gives an overall loss in elevation over the run.

Back on topic, my GPS struggles for signal for the first mile after leaving the office (corridor of tall buildings) but the overall distance is accurate so it must record the first mile using the speed sensor?

Karl2010

It does take the speed/distance from te GSC10! Only when a GSC10 is NOT present it will take it from the GPS...

If you wanna know more go tot he Garmin Forums. Very usefull!

think its

www.garmin.com/forum
or
forum.garmin.com

cant remeber

morstar

Off topic as my phone gps is not garmin...

however. For distance I have a very small percentage error between my gps and computer which I am happy accepting as the small variation between autostop settings.

Altitude and top speed readings are extremely generous from the GPS though. Or cloud cuckoo land for want of a better description.

xraymtb

Karl2010 wrote:

It does take the speed/distance from te GSC10! Only when a GSC10 is NOT present it will take it from the GPS...

For the Edge 305, the GPS takes precedence according to Garmins FAQ and manual. On the Edge 705, the GSC10 takes precedence (although there is an ongoing thread on the Garmin forum that seems to dispute this at present).

Cleat Eastwood

i did the liverpool chester l'pool 90miler and used a garmin and a wireless comp. There was a 5 mile difference in the final readings. I can only assume it was caused y loss of satellite contact, even though the data downloaded shows a continuous ride.

daviesee

As I will not be using a turbo trainer, I did not fit the wheel sensor, thinking that the 705 GPS made it redundant.

Should I fit it?

xraymtb

Fit it just to get the cadence reading if nothing else.

blorg

daviesee wrote:

As I will not be using a turbo trainer, I did not fit the wheel sensor, thinking that the 705 GPS made it redundant.

Should I fit it?

I have the sensor on three bikes but not the other seven. IMO it makes little difference (other than giving you cadence of course.)

daviesee

blorg wrote:

daviesee wrote:

As I will not be using a turbo trainer, I did not fit the wheel sensor, thinking that the 705 GPS made it redundant.

Should I fit it?

I have the sensor on three bikes but not the other seven. IMO it makes little difference (other than giving you cadence of course.)

Thanks!

PS:- I have the magnet fitted to the crank and the sensors fitted (for cadence) but not the wheel magnet.

xraymtb

As an update from the Garmin forum:

One guy did a bunch of tests setting custom wheel sizes etc. on the same course and has shown without a doubt that the 705 uses the wheel sensor to calculate speed and NOT the GPS.

That said, when he set ridiculous sizes for the wheels (9000mm and 90mm), the unit appeared to use the GPS as the distance travelled didnt change. With a much more reasonable wheel size, it did change by the amount expected for the change in wheel diameter.

Karl2010

The edge does take the speed/distance from the GSC10, FACT!

Take your bike outside. Turn on Edge, wait for GPS.

Pick the back wheel up so its not touching the floor and spin it. Look at your Edge and you'll see it displays a speed. If not your GSC10/magnets need adjusting

daviesee

Karl2010 wrote:

The edge does take the speed/distance from the GSC10, FACT!

Take your bike outside. Turn on Edge, wait for GPS.

Pick the back wheel up so its not touching the floor and spin it. Look at your Edge and you'll see it displays a speed. If not your GSC10/magnets need adjusting

Take your GSC 10 off.

Ride the bike.

GPS displays speed and distance. FACT!

So what's the conclusion?

Seriously?

jibberjim

daviesee wrote:

Karl2010 wrote:

The edge does take the speed/distance from the GSC10, FACT!

Take your bike outside. Turn on Edge, wait for GPS.

Pick the back wheel up so its not touching the floor and spin it. Look at your Edge and you'll see it displays a speed. If not your GSC10/magnets need adjusting

Take your GSC 10 off.

Ride the bike.

GPS displays speed and distance. FACT!

So what's the conclusion?

Yes, as has been said, It uses the speed from a Speed/Cadence sensor in preference to GPS if it's available, if it's not available, it uses the GPS. If you added a powertap in there it would get even more complicated with Cadence coming from the GSC 10 and Speed from the PowerTap.

If there's only one source, it uses it, if there's more than one it has a preference for a particular source which should be the most accurate.

owenlars

Mike400 wrote:

Slight thread Hi-jack

Ive an edge 305, and the elevation data is next to useless.

I can do a loop and when I return to the point I started from according to the edge Im 100ft lower......

When I ride into work it records the run as downhill all the way (correct) but on the way home it also records it as downhill even though there should be an overall gain.

Weirdly big hills show in the right place on the graph, but the starting altitude is always waay higher than the finish altitude regardless of where I cycle which gives an overall loss in elevation over the run.

Back on topic, my GPS struggles for signal for the first mile after leaving the office (corridor of tall buildings) but the overall distance is accurate so it must record the first mile using the speed sensor?

The 305 and 705 both use atmospheric pressure as well as GPS altitude measurement. GPS altitude is pretty innacurate and as you know barometric pressure changes. If pressure goes up the machine will think you are lower and vice versa. For this reason as a measurement of absolute elevation they are not good, however as a measure of change in elevation they should be pretty accurate most of the time. If you look in the Garmin Connect web site you will see there is an option to adjust GPS elevation data.

daviesee

jibberjim wrote:

daviesee wrote:

Karl2010 wrote:

The edge does take the speed/distance from the GSC10, FACT!

Take your bike outside. Turn on Edge, wait for GPS.

Pick the back wheel up so its not touching the floor and spin it. Look at your Edge and you'll see it displays a speed. If not your GSC10/magnets need adjusting

Take your GSC 10 off.

Ride the bike.

GPS displays speed and distance. FACT!

So what's the conclusion?

Yes, as has been said, It uses the speed from a Speed/Cadence sensor in preference to GPS if it's available, if it's not available, it uses the GPS. If you added a powertap in there it would get even more complicated with Cadence coming from the GSC 10 and Speed from the PowerTap.

If there's only one source, it uses it, if there's more than one it has a preference for a particular source which should be the most accurate.

All fair enough and perfectly understandable.

The question from my point of view though is this:-

What is the more accurate for speed and distance?
Me rolling down the drive staring at a measuring tape for calibration with potential parrallax error or the GPS?

Cleat Eastwood

This is from the manual:

"There are 2 sensors on the gsc10, one for cadence and one for speed. The speed data from the gsc10 is only used for distance calculation when GPS signal is weak or GPS is turned off."