Garmin Edge 705 Wheel Size

[Andrew_P]

OK, would any of you mind taking a look at how your Garmin is set up for wheel size?

I changed my Tyres from Specilaized Roubaix Pro 700x x 25mm to GP4000 700x23

The GP4000 box came with a handy rolling dia on the side of 2122 so I thought I would be clever and change it in my Garmin from Auto (2025 for the Roubaix when I opened the Bike profile)

And voila I am now superfast and my commute has gone from 14.70-14.74 to 15.03 this morning!

The superfast is not all down to the gaming of the wheelsize as my time was my second fastest ever, but there is a north wind!

I am just trying to figure out what is best, I do not really trust the Auto Size as I would say the Roubaix Pro were a fair bit taller than the GP4000 as I now have enough clearance to try and fit some Cruds, but I also think with my weight on the bike the rolling dia will be less than 2122..

 

[martint235]

Does the Garmin need wheel size? It's a GPS device and measures your speed through the GPS system. If your 705 is reporting that increase in your average speed then I think you should just be giving yourself a good pat on the back!

 

[Andrew_P]

I use the GC10 Cadence & Speed sensor that needs a Wheelsize. Recently I knocked the GC10 and it wasn't picking up the wheel sensor and my speed was up and down and in fairly long parts of my ride zero, it knocked my commute down to 13.5 miles. I assumed this was poor Satelite reception

My edge seems to be playing up a bit lately my Accent and Climbs are all over the place started one ride at -300ft

 

[martint235]

I'm not sure about the altitude bit but that sounds like it could be a dodgy unit as iirc the 705 has a built in altimeter so isn't depending on the maps.

I've never used a GC but it sounds a strange way of measuring speed on a GPS enabled device. I only had a 605 though (and now an etrex) so probably best if we wait for someone who owns a 705 and a GC to pop along.

 

[fossyant]

I leave mine on auto. It occasionally bleeps "wheel size calculated" at the start of a ride within half a mile of the house. Used with GSC10.

I use the garmin on 3 bikes, 2 with GSC 10's.

 

[Gary E]

The 705 can calculate distance using GPS data but the wheel sensor is more accurate.

Look at it this way - the GPS can calculate your position within a few metres and so can work out what road you're on but it doesn't know if you take a tight line round every bend or a wide one. Over the length of the ride this can add up to quite a difference.

As for the auto setting it's simply counting the wheel revolutions over a measured distance (using GPS) to calculate for itself the wheel diameter.

GPS signal can drop out due to environmental conditions leading to incorrect readings, the sensor is more reliable.

I'm not 100% sure but I think the 705 will always use sensor data first if it's available but default back to GPS when it isn't.

 

[amaferanga]

2096 should be about right for 700x23. The Garmin auto thing is not reliable unfortunately - much better to manually enter the value. Leaving it on auto you can (and people do - look on the Garmin forum) get unreliable wheel circumference values. At least if you set it to a known value and have your tyres at the same pressure all the time you know that the one thing you have is consistency - it may be off by a little, but it'll always be off by very, very nearly the same amount. Not so with a value that's dependent on the quality of the GPS signal you're getting and the cleverness of Garmin developers.

The good news is that 2025 is almost certainly an underestimate for the 700x25 so you've been going faster all along. 2122 seems a bit high for 700x23.

 

[Andrew_P]

2096 should be about right for 700x23.

Edited the quote, but all of your post made made a lot of sense, thanks. I read on a forum how to get the auto size to start a fresh, and it defaults to 2100 before you start off, by the time I got home it was 2075, So as you say I think I will just set it at 2096 just so I know it is the same each ride.

 

[Mark Grant]

On a ride a while ago the readout on my 705 was all over the place. I turned off the cadence function and it was all good.
It may just need a new battery in the sensor but I haven't got round to trying a new one yet.

 

[Gary E]

When I bought my Garmin Edge 705 I left my Cateye Velo on the bar as well just to compare things. After a 60 or 70 mile ride I can honestly say that they always agree within a tenth of a mile or so. My Edge 800 is the same when used on the same bike (same sensor) so I'm quite surprised to hear others are having these problems.

I've had problems with both units with the heart rate strap though (can't win them all I suppose!)