I’ve got an Edge 500 and a mate has an older unit, I think a 705.

We did a ride at Thetford yesterday & at the end, his had covered 17.8 mile, whereas mine was 15.6.
Quite a discrepancy…

I haven’t looked at the plot yet, but it normally doesn’t have any problem keeping up under tree cover.

Obvious differences are that I have my cadence sensor fitted (not that I really use it, just thought I might as well It it) and I have a 2nd set of wheels for the road with the speed sensor on. I have 3 bikes set-up; inbred road, inbred off road & FSR. I had set it to inbred off road & as the speed sensor isn’t on those wheels, it can’t be using that to measure distance; it must be gps.

My mate is convinced that his is right from comparisons against lap lengths at races, and distances covered on the road compared to his car…

I should check mine in the car & perhaps even against My Tracks on my phone. I also need to look at the plot to see if it has straightlined sections.

But, in the mean time…any ideas?

occam’s razor says you need to look at the plot to see if it has straightlined sections

True, but I got back late from my ride & couldn’t be bothered…

I can’t recall it ever straightlining a route though. I’ll check that and perhaps take in with me in the car to work tomorrow.

Could it be something to do with recording frequency. you are recording every second he is not or the other way around.

On my 705 I know this is a setting you can change to save space when recording. I don’t have a 500 but it may have a similar setting.

Thetford has trees with plenty of coverage on trails. Newer units theoretically work better in those situations. Compare the two on a ride with plenty of sky visibility.

http://cincyhalfmarathon.com/GPS%20Accuracy.pdf

whilst all those factors are a possibility its also worth checking his GPS is set in (statute) miles not nautical miles as the difference between the two numbers is pretty close to the error you would get like that.

Thetford has trees with plenty of coverage on trails. Newer units theoretically work better in those situations. Compare the two on a ride with plenty of sky visibility.

This.

Newer GPS chips are more reliable/powerful than older.

Is one of them set to only record when moving?

Dunno if it’s relevant but since the last firmware update, my Edge 500 has developed a rather annoying habit of autopausing and resuming as the mood takes it. It only seems to do this when fitted on my cyclocross bike and not when fitted on my mountain bikes. Potentially relevant is that cyclocross bike has speed/cadence sensor fitted, mountain bikes do not…

Blades2000 – not sure about frequency of recording. I think mine is set to an intelligent mode where it changes frequency depending on how often direction & speed changes are made.

cp, mine is the newer unit but my mate is pretty sure his unit is consistently accurate. I did wonder whether the tree cover was ballsing things up, but we often ride there & I haven’t noticed it missing data.

Oooh – good idea, will check mile setting!

Bruce – yeah, mine has autopause, mates doesn’t. Did wonder if that could be a factor…

Hopkinsgm – I haven’t updated firmware yet. But I did wonder if running it sometimes with a speed sensor on one set of wheels (on the road) and then a different set of off road wheels with no speed sensor might balls things up. But, isn’t that the whole point of those 3 profiles you can set…?

Not gonna get the chance to upload the route, as off out with the missus, but will take it on car journey tomorrow perhaps & see what happens…!

Autopause is a big factor and will affect the distance massively.
On mtb trails its much more accurate to measure off a speed/cadence sensor than from the GPS plot due to the straightening of sharp corners. I don’t think it matters as much on the road as most are less convoluted compared to mtb trails.
I don’t bother with speed/cadence sensors on my road bike but all my mtb’s have them.

Have just had a look at the settings…

You can only set to miles or km, so that’s ok…
The bike details setting has the option to select what sensors are on the bike…
So the Inbred Road is set to spd/cad sensor as I have the magnet on the road wheels, while the Inbred Off-road is set to no sensor, & doesn’t record cadence info and has no magnet to get speed/distance info from…

So I don’t think it’s that…

I wonder if it’s autopause…easy enough to turn off & try.

Hugor, your logic for speed sensor is a good one. I thought it would be better to have it on the road, rather than off road as my cadence should be more consistent. But I see what u mean about distance – perhaps I should get a magnet for both wheels…
Thing is, I’d expect both units to display a similar inaccuracy as averaged out over the route, I would think the ‘wobble’ would be about the same for both units…

I noticed my 800 was reading 0mph the other night, when I was clearly moving. Turned out the magnet had moved and the speed sensor had stopped working, after 5 mins or so, it reverted back to using the GPS for the speed. It still tracked the route accurately, so I may export it as a gpx file and see if another program like endomondo thinks I’ve done a different distance.

Used it in the car this morning…46.2 miles on the Garmin compared to 46 indicated on the car. So, accurate enough.

Differences:

No tree cover
Very few auto pauses
No cadence sensor to cause ‘confusion’.

Next things to try are turning off auto pause and trying again on the Inbred and using it on the FSR that has no cadence sensor.