# Garmin Speed Sensor Wheel Size Setting?



## bpsmith (17 Oct 2015)

Do you guys set the wheel size for your Garmin Speed Sensors to auto or manual? What size do you put when running 700c x 25?

Set up new Garmin 520 today. Did a ride and set it at auto. Checked on getting home and it's set to 2096. For 25mm tyres the manual say 2105 and that's how I had it with old 510 model.

Just measured in my garage and it's actually 2160. Can I assume that this is due to the tyre fitted to the wheel and the wheel itself would be more like 2105 or should I be using 2160?

If I should measure without the tyre fitted, then why the different measurement for different tyres?

2096 compared to 2160 is almost 3.1% or 0.5mph higher. Don't want to set it higher than it should be.

Edit: Clearly the wheel without tyre would be a lot more than 3.1% difference?


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## bikeman66 (17 Oct 2015)

bpsmith said:


> Do you guys set the wheel size for your Garmin Speed Sensors to auto or manual? What size do you put when running 700c x 25?
> 
> Set up new Garmin 520 today. Did a ride and set it at auto. Checked on getting home and it's set to 2096. For 25mm tyres the manual say 2105 and that's how I had it with old 510 model.
> 
> ...


I can't imagine that you should be measuring the circumference of the wheel without the tyre. Surely that would leave the whole deal open to the vagaries of tyre size. My default method of measuring circumference, which I guess most folk use, is to put pen mark on the side wall of the tyre in the down position closest to the floor, make another corresponding mark on the floor, wheel the bike forward so that the pen mark on the side wall ends up back in the down position having completed one full revolution, make another mark on the floor. Measure the distance between the two marks and this has to be the definitive measurement of the circumference of your wheel.


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## bpsmith (17 Oct 2015)

bikeman66 said:


> I can't imagine that you should be measuring the circumference of the wheel without the tyre. Surely that would leave the whole deal open to the vagaries of tyre size. My default method of measuring circumference, which I guess most folk use, is to put pen mark on the side wall of the tyre in the down position closest to the floor, make another corresponding mark on the floor, wheel the bike forward so that the pen mark on the side wall ends up back in the down position having completed one full revolution, make another mark on the floor. Measure the distance between the two marks and this has to be the definitive measurement of the circumference of your wheel.


Just edited my post to say that it can't be without the tyre. Your reply came through straight after, confirming this.

I measured mine by lining the valve up with the start of a carpet tile in my garage and putting a key on the floor pointing at where the valve ended up, after one full revolution. Then measured point to point. Same idea as your suggestion. Good to confirm it and will manually set at 2160 for next ride.


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## HarryTheDog (17 Oct 2015)

I went through the same thing with 25mm gatorskins, the box they came in said circumference 2113, garmin set to Auto I got silly numbers every time around 2086. I measured it by rolling the bike on the patio with a builders tape meaure and rolled it with my wieght on it over 7 revolutions . I did this 3 times and got a average , divided by 7 and and got 2111.
On the web sheldons site etc like you I found 2105. I have set it manually to 2111. However I think the garmin still records rides by gps as on my ride I see some good speeds. When I upload to garmin site those speeds dont appear they are 1 to 3 mph slower. Annoys the hell out of me.


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## bpsmith (17 Oct 2015)

That would annoy me too @HarryTheDog !


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## HarryTheDog (17 Oct 2015)

@bpsmith sometimes the stupid gps works in your favour, on thursday it recorded





yeah 141 mph, I dont think so, sort your sattelites out you stupid box.


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## Milkfloat (18 Oct 2015)

On the 520 I leave it on auto, if switching to another bike/wheel size combination it can be a little bit squiffy for the first few hundred meters but generally ok. The GPS far more accuate with GLONASS enabled, so the speed sensor is not hugely needed.


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## bpsmith (18 Oct 2015)

Milkfloat said:


> On the 520 I leave it on auto, if switching to another bike/wheel size combination it can be a little bit squiffy for the first few hundred meters but generally ok. The GPS far more accuate with GLONASS enabled, so the speed sensor is not hugely needed.


Of course. I would have to agree with that, but the speed sensor overrides the GPS when connected.


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