# Wheel size - Auto or Custom? (Garmin GPS set up)



## D-Town (Oct 20, 2005)

Just picked up the new Edge 500 and I like it so far. I only have two rides on it so the jury is still out. I really like the size of the unit and how it fits on to my stem. The temperature is pretty cool though it takes a little while to "settle" to the current temp.

My question is setting up the wheel size for my bike. So far, I chose to have the GPS auto calculate the wheel. My average speed and distance has been slower when I've ridden with my old computer along side of the GPS. Is it better to get a true measurement of my tire and then enter that value into the unit?

On another note, does the setting for bike weight ever come into play with the speed or distance? I'm guessing that it is really a factor for total calories burned.

Thanks!


----------



## Harold (Dec 23, 2003)

you're a new adopter...I don't think many others here have that GPS yet. The other Garmin cycling computers don't have a bike weight option. I would guess it is a calorie computation feature. As for wheel circumference, I've always gone the manual route with any computer I've used. The auto route makes a lot of assumptions about tire size and pressure. Manual calibration removes those assumptions.


----------



## slocaus (Jul 21, 2005)

D-Town said:


> Just picked up the new Edge 500 and I like it so far. I only have two rides on it so the jury is still out. I really like the size of the unit and how it fits on to my stem. The temperature is pretty cool though it takes a little while to "settle" to the current temp.
> 
> My question is setting up the wheel size for my bike. So far, I chose to have the GPS auto calculate the wheel. My average speed and distance has been slower when I've ridden with my old computer along side of the GPS. Is it better to get a true measurement of my tire and then enter that value into the unit?
> 
> ...


Wheel size:
I always did rollout on my regular computers. With the Garmin, the GPS will figure speed / distance on its own, and only use the settings if you use the cadence / speed sensor and turn off the GPS. On my road bike with the sensor, I tried setting the wheel size way small (20" wheel and not 29x53) to experiment. Only with the GPS off did it read way short. So unless you use it on a trainer or turn the GPS off all the time, it will not matter. Do a roll out with tires inflated to standard pressure, sit on the bike, roll it out and measure. My guess is that the computer you have now is not calibrated accurately.

Bike weight:
The _Bike Profiles_ on my 705 have a place for bike weight in each one. I like that you can also rename the profiles, so I have "Canzo", "Inbred", and "Lobster" for mine.


----------



## 23mjm (Oct 22, 2005)

NateHawk said:


> you're a new adopter...I don't think many others here have that GPS yet. The other Garmin cycling computers don't have a bike weight option. I would guess it is a calorie computation feature. As for wheel circumference, I've always gone the manual route with any computer I've used. The auto route makes a lot of assumptions about tire size and pressure. Manual calibration removes those assumptions.


My 305 has a place for bike weight on the bike profile page!!! As far as using the Auto wheel size function--I have always just uses it, It make no assumptions on tire size and pressure, what it does it count the tire revolutions in a given distance about .5 miles/1km and the does some math to give you the tire diameter in millimeters. I find if I do the same 25 mile ride I get the same distance on the GPS to with in 3 or 4 1/100 of a mile!!! I only use the speed/cadence on my road bike and just the GPS speed/distance on my MTB's. One more thing every time you change the bike profile it recalculated the wheel size. Even with the recalculations it has proven very consistent over the 3 years I have owned my 305.


----------



## AnthemRider (Feb 7, 2007)

NateHawk said:


> you're a new adopter...I don't think many others here have that GPS yet. The other Garmin cycling computers don't have a bike weight option. I would guess it is a calorie computation feature. As for wheel circumference, I've always gone the manual route with any computer I've used. The auto route makes a lot of assumptions about tire size and pressure. Manual calibration removes those assumptions.


Most cycle computers make assumptions about wheel size when you use "auto size," but my understanding with Garmin is that the GPS unit tracks your speed and compares it with wheel revolutions and then stores the calculated wheel/tire size -- so is likely to be as accurate if not more accurate than if you measure it yourself.


----------



## Harold (Dec 23, 2003)

AnthemRider said:


> Most cycle computers make assumptions about wheel size when you use "auto size," but my understanding with Garmin is that the GPS unit tracks your speed and compares it with wheel revolutions and then stores the calculated wheel/tire size -- so is likely to be as accurate if not more accurate than if you measure it yourself.


Interesting. I've never heard that before.


----------



## slocaus (Jul 21, 2005)

AnthemRider said:


> Most cycle computers make assumptions about wheel size when you use "auto size," but my understanding with Garmin is that the GPS unit tracks your speed and compares it with wheel revolutions and then stores the calculated wheel/tire size -- so is likely to be as accurate if not more accurate than if you measure it yourself.


Any more info on how it "*compares it with wheel revolutions*"? I can see how it tracks speed, but how does it count wheel revolutions?


----------



## Timon (May 11, 2008)

slocaus said:


> Any more info on how it "*compares it with wheel revolutions*"? I can see how it tracks speed, but how does it count wheel revolutions?


with the magnet that you put on the rear wheel when you install the cadence/speed sensor.

if it knows those revolutions, and knows the distance you travel in a specific amount of time, then it can simply calculate the circumference of the wheel.


----------



## AnthemRider (Feb 7, 2007)

slocaus said:


> Any more info on how it "*compares it with wheel revolutions*"? I can see how it tracks speed, but how does it count wheel revolutions?


The GPS measures how far you've traveled. If you go 2 meters between each time the magnet on the wheel passes by the sensor, I bet most units have enough computing power to figure out the circumference of the wheel/tire.

Although to be fair, in my first post I should have said compared distance, not speed. Dangers of quick internet postings.


----------



## slocaus (Jul 21, 2005)

I misread that the GPS somehow calibrated wheel size without the sensor and magnet, explain that! With the magnet sensor, it is a *duh*, but thanks for explaining the obvious. I just misread the other part. Lack of sleep or caffeine or both.


----------



## D-Town (Oct 20, 2005)

Thanks everyone for the info!


----------



## dk13 (Apr 11, 2011)

I was playing around with the same thing, but the strange thing is every time it gave me a different wheel size, totally different than the previous time and my rollout measurement.

According to tire table my kenda navigal 2.1 should measure 2098mm... After a no weight rollout I got 2096mm with my weight on top I did 3 rollouts and average out to 2049... Than I set to auto on the garmin and doing straight runs I got the following sizes: 2006, 2053, 2019, 2038, 2012, 2072... Which averages out to 2033mm

Now here is my question is should I go with my rollout or the average auto?

And the other question is once it detected the wheel size on auto, does it keeps correcting itself, so the more you ride the accurate it gets or it just stops calculating it?

Cheers 
Dan


----------



## nov0798 (Nov 27, 2005)

Im not sure how you are getting different rollout measurements using the same wheel/tire combo? Use the chart in the back of the book, and enter that number. Thats it, dont over think this. Also check out forums.garmin.com for more info.

if you want the unit more accurate, use the wheel mounted magnet and the gsc-10 for speed/cadence. If you are trying to use the custom size with the gps, it wont work. Its either one or the other GPS or GSC-10, not both.


----------



## dk13 (Apr 11, 2011)

nov0798 said:


> Im not sure how you are getting different rollout measurements using the same wheel/tire combo? Use the chart in the back of the book, and enter that number. Thats it, dont over think this. Also check out forums.garmin.com for more info.
> 
> if you want the unit more accurate, use the wheel mounted magnet and the gsc-10 for speed/cadence. If you are trying to use the custom size with the gps, it wont work. Its either one or the other GPS or GSC-10, not both.


well, rollout with weight on the bike does make a difference, since the tire will get squashed a bit... but using the auto setting on the garmin it uses the GPS to track the distance and counts how many passes the magnet did, so after 500m or so you will get a beep saying wheel size calculated... than when you go to bike info, you will see the wheel size it calculated... every time i reset and do a new run, it gives me a different wheel size... completely off from the previous one.... i know GPS is not the most accurate reception, and you will get a difference of 10-20m, but knowing that you would assume that the garmin would use a longer than 500m run to be more accurate... but still doesn't explain why there is such a big difference between runs...

but i guess i'll just stick to the rollout number 

cheers,
dan


----------



## fueledbymetal (Jul 20, 2007)

Go with your weighted rollout value. I tried the auto when I switched to a 29er and it was definitely off.


----------



## obie.miller (Mar 11, 2008)

I am using my Edge 705 with a Powertap. Apparently the powertap acts as a cadence sensor because my speed is off unless I enter the correct wheel size. 

My question now is, if I'm using the Garmin on a different bike with no sensor, wheel size doesn't need to be changed for that bike (26er, 29er, 700c, etc), correct?


----------



## KINBOY (May 23, 2004)

obie.miller said:


> I am using my Edge 705 with a Powertap. Apparently the powertap acts as a cadence sensor because my speed is off unless I enter the correct wheel size.
> 
> My question now is, if I'm using the Garmin on a different bike with no sensor, wheel size doesn't need to be changed for that bike (26er, 29er, 700c, etc), correct?


You are correct. It will only use the value if the sensor is sending signals.


----------



## Bllocky (Nov 2, 2011)

*Tyre size reference from Garmin 705 manual*

Hi on pages 62-64 of the 705 manual which can be found in pdf format on the web it gives tyre sizes, I guess these are indicative but worth a look.


----------

