My partner had been using a Bolt on her Trek along with the Duotrap S speed/cadence sensor while I use the Edge 830 with Garmin Speed 2 and Cadence Sensors on my bike. We noticed that for every single ride that we do together, and I am counting dozens, the Bolt always registers 0.2 to 0.4 miles (most often 0.2 miles) less total distance on ~ 25 mile ride. We have both computers set to autocalibrate wheel sizes. We have seen that difference on both longer and shorter rides. Although the consistent difference (rather than a random difference due to precision) bothered me a bit, I just attributed it to differences in the pairing of the computers with the different sensors on the two bikes.

But recently I was able to compare both computers paired with the exact same sensors (Garmin Speed/Cadence 2) on my bike for close to a dozen rides. The difference of 0.2 to 0.4 miles (Bolt always lower) on a 25 mile remain still remains even using the same bike and paired sensors. I really don't have a possible explanation except that it is something regarding the difference in the autocalibrate functions on the two computers. I guess the next thing to try when I get a chance to ride is to enter a fixed and same value for the wheel calibration for both computers and see what happens.

In the meantime, any ideas on what is causing the difference? In reality, I don't know which is "correct".

Thanks.

1% disagreement is too much for you?

The sensors just transmit wheel rotations. It’s not likely to be the sensors.

I believe the Garmins measure the wheel circumference at the beginning of every ride.

You should try the two computers on the same bike.

They are both “correct” with some error.

They both might be “wrong”. That is, one might not be more correct than the other.

All you really know is that they are different.

​​​​​​You're exactly right. Both computers auto calibrate against GPS. The trouble is that GPS is very "noisy" and although it's very good, it's never perfect. Both the Garmin and the Bolt have programming to smooth out some of the noise in the GPS data, sightly differently, resulting in the difference you see. It isn't that one is right and the other is wrong, it's that both are very very close and one is a little more so.

Does one or both use auto-pause? If so, are they set to the same start/pause threshold? One could lose a few at every stop sign if the threshold number is set high.

Do a roll-out measurement on both bikes and compare those numbers with the autogenerated ones.

Use the ones that are "better".

The Garmin lets you plug-,in your own number. Not sure about the Bolt.

Using Auto-Pause or not, would show a ride time discrepancy not a distance discrepancy.

I'm not aware how the on bike sensors work which the OP seems to have. I don't use them except on a trainer otherwise I am GPS only.

I know that if I turn the unit on in the driveway and leave it for very long, it racks up distance due to signal drift. I guess that I need to save a 30' "trip" in the driveway and see what the ride distance is.

Most assuredly, neither is correct. We only know the differ by 0.8% from each other.

The most important question? Which one gets you those PB on Strava.

That 0.8% difference is likely closer than if both units only used GPS only for distance, no wheel sensors. Maybe not with a clear sky signal, but forests, buildings, etc, can throw off the calculated position quite a bit.
~~~
The Garmin shows the auto calculated wheel size in the menu. "hamburger menu icon" --> sensors --> select the wheel sensor --> sensor details. It only shows the wheel size when the sensor is active.
It would be interesting to see how often this changes during a ride. It should stay at the same value, but I'm guessing it drifts a little.
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That 0.8% discrepancy between the Bolt and Garmin would be about 16mm for my 2093 wheel size.

"A man with a watch knows what time it is. A man with two watches is never sure."

Seems to apply to GPS devices, too.

That probably is fast becoming non-sensical to many today as they grew up using time pieces that are network connected and always show the same time.

I've used that phrase plenty of times too, but I think it's starting to show our age.

Perhaps we should just modify it for today and substitute GPS and distance for watch and time.

The guy didn't say it was too much, and he didn't ask how to fix it. He asked how to understand it.

I knew that from the very start. Do you have an overwhelming need to respond to obtuse replies? <grin>

I have a friend that always has two gps units and he never knows where he is. That's one reason I got my gps, I got tired of him thinking he was lost.
Back before gps gained widespread use, I have actually heard it said that someone with one navigation system is never lost, but someone with two navigation systems never knows where he is.

Somebody told me once how weird it is that a mile is so close to two thousand steps. Mile from mille, for thousand; like millennium or millimeter. A mile is a thousand paces, a pace is a step with the right foot and a step with the left. In the very old days, before GPS, if you needed a long distance measured you went to a professional walker and counter. Using that very crude technology and the angle of a shadow at noon, ancient people were able to figure out the size of the Earth to within a few percent.

Neils Bohr was a jerk as a teenager, like everyone. His science teacher failed him. On an exam, the teacher had asked how can you use a barometer to measure the height of a tall building? Everyone with a Garmin knows the answer, and complains sometimes about the result. Bohr answered "take the barometer to the roof of the building, drop it to the ground, measure the time it takes, and calculate the distance by assuming constant acceleration under gravity." The teacher held young Niels after class the next day and said he would pass the child of he can demonstrate that he learned the course's material by giving the correct answer. Bohr relented: "you can learn the height of a tall building using a barometer by building a gyroscope out of the barometer and measuring the strength of gravity at the ground and roof, and then calculating the distances. Or you can lower the barometer to the ground and measure the amount of rope required. If the building has a superintendent, you can trade the barometer for accurate information regarding the height of the building."

Probably not a true story.

That story sounds like it was about an adult Richard Feynman. Although people like that tend to be blunt.

The teacher went on to work for Garmin.

https://shahidulnews.com/the-barometer-story/

This make no sense.

Your friend would likely have been equally "lost" with one device.

​​​​​

You realize that many times there is quite a bit of fisherman tales to our conversations don't you?

Maybe you are just trying to build on that, but it's hard to tell.

I would make sure both devices are using the same GPS systems. I am sure the problem is with the Russians.

What's that old saying about a man with one watch knowing what time it is, but a man with two watches can never be sure?


Edit: whoops, someone else beat me to it. Should read the whole thread before I post.

This one is so old it stinks.

The post doesn't really read as a "story" either.

I am not sure about the Wahoo but for my Garmin 1030 the calibration value get stored and available. I’ve compared it to the actual measured why’s circumference. They were different but the measured circumference matched a local calibrated mile.

As for you distance difference I’d offer that your gf may cut corners closer then you. This may add up and partially explains why her about 25 mile distance is shorter.

Otherwise GPS locations we have on our units are not very accurate. It something like 30 feet.