Speed and Distance way off from Garmin

[slp3939]

The weather is finally getting nice enough to start biking outside. I've done a calibration ride and everything looked good on that. When I went for a longer ride this last weekend the data on the Newton was way off. I've attached the cal ride and the ride that seems way off. My Garmin was reading the following information:
Distance 40.48 miles
Avg. Speed 20.94
Avg. Cadence 93
The cadence seems pretty close but everything else is is way off. The Newton seemed to consistently read about 60% of what my Garmin was showing. I thought maybe it had to do with the wheel circumference but I would have needed to be way off to get results this different. I've done a rolling test to get that measurement and I'm coming up with 82.5 inches which is 2095.5 mm. I've also put the bike on my trainer and just spun the wheel and moved the magnet around. It doesn't seem to matter where I put the magnet as long as I'm getting a reading it's showing way less speed than the Garmin will show. Someone please help.

[Velocomp]

Set Isaac filtering to zero and you'll see that your speed sensor is not working properly.

Reposition the sensor/magnet according to the instructions and all should be well

[slp3939]

I've changed the filter to 0. I've been moving the speed magnet up and down the spoke. I'm able to get speed readings on a consistent basis however it is still consistently about 60% less than the garmin is showing. I've tried the magnet that came with the newton and used other magnets from old bike computers. They are all showing similar results. I've attached a few pictures however they may be hard to see. My cadence and speed sensors look different than the ones shown on the YouTube set up video. I've tried everything I can think of with no success. My cal ride and all rides on the trainer prior to my last ride appeared to be working correctly. Please help.

[Velocomp]

The photo reveals the problem. The spoke magnet is positioned improperly. It should pass by the notch in the speed sensor, with no more than 1-2 coin widths of separation.

Move the magnet and you will be OK.

[slp3939]

I've moved the spoke magnet and put it along the notch on the speed sensor. I'm still getting the same results. I've slowly moved the spoke magnet along the spoke so it passes by different locations on the speed sensor. I've also adjusted the speed sensor so that gap is as small as I can get it without them making contact. It's giving a speed reading every time. The reading it's giving me however is still way lower than it should be. It seems to be a consistent ratio less than what the Garmin is showing. It appears to be acting like my tire size is way off but I've checked on isaac and my wheel circum is set at 2095 which even if it's off by 10mm I wouldn't think I would get results so far off.

[Velocomp]

Please do a short ride with the correct sensor/magnet position and post the ride file

[racerfern]

But is the wheel magnet within one coin thickness away from the sensor. Not left to right but actually spaced away. It appears to be a lot further in the photo.

[rcmaps]

I know what the problem is. You mentioned setting the wheel size in Isaac but did you actually set the wheel size in the Newton itself or "send profile to Newton"?

I suspect not as your ride file posted above has your wheel size set as 2088mm instead of 2096mm.

Change that and it should match Garmin's speed.

[texmurphy]

You don't say which Garmin nor what the wheel size the Garmin thinks you are using.
If you let the Garmin do "Automatic" wheel size then you can get Garmin speed variances.
Garmin should be set to a "Custom" wheel size using your roll out.

[Pete]

Cross-check with the gear ratio in the ride data.
It should match your bike gearing if calcutated speed is correct.
Eg my son on his road bike he gets either a gear ratio of 3.00 when in the 48x16 or 3.20 when in the 48x15 which confirms (650c) wheel at 1954mm is correct.

If you have a 53/39 chainset go for a ride in 39x13 and you should get a gear ratio of 3.00
Or for compact 50/34 hopefully you can select 34x17 or 50x25 and get a gear ratio of 2.00
^ use these even dividers to avoid rounding issues.

[slp3939]

In response to some of the previous post:
The magnet is as close to the sensor as I can get it.
The wheel size profile I changed after the ride. The difference of less than 10mm wouldn't produce the difference the speed and distance were showing.
The Garmin I'm using is a Forerunner 305. It's basing everything off GPS not wheel size.

Now to my ride last night:
Before I went on the ride I was messing around with the Newton and found a section called profile 1. I clicked to select that even though I had sent that profile to the Newton thru Isaac previously (or at least i thought I did). When I began my ride the speed appeared to be accurate now but my watts were showing in the 1000's. After about 10 minutes that settled down and appeared to be showing me real time accurate data. I'm not sure if it did some sort of internal cal ride or what. After about a 30 minute ride I stopped and did a new Calibration ride. I've attached both rides. Everything appears to be working as it should now. If anybody sees what was wrong originally or was different from this ride compared to my original post please let me know so I don't do it again.

Thanks everybody for their quick responses and help.