Distance covered comparison F945 vs Cyclometer

cyclometer?  like a magnet/wheel sensor to a bike computer?    A few things can lead to a wheel sensor being much higher than a GPS.  One is many turns, each turn you loose just a little bit of distance as the gps cuts the corner due to averaging.  If biking in any sort of loose gravel or sand leading to tire slip, gives you distance to wheel sensor... but you don't move.  If it was a very long ride many turns and small amount of wheel calibration can add up!  Auto pause on GPS will also lose you distance at each stop/start.  Also look at  the GPS track did it start with a good reading?  On the road and near where you hit 'start' sometimes the first couple Km's can be jacked up and have you way off track.  Same with if you are deep in the woods or a large city or tunnels.

If none of these apply... the most obvious is that I would say the wheel size is off slightly in cyclometer and needs calibration.  

I linked my speed sensor to F945 and wheel size was set properly ... same with cyclometer... but when i tested both at the same time on  almost 3 rides ... the diff is within 2-3 kms giving cyclometer a higher distance covered.

Did you set the wheel size to the size of the wheel or did you measure the wheel size by sitting on the bike with your usual tire pressure and rolling at least a full revolution of the wheel (two revolutions will get an even better result)? The actual wheel size during load will be smaller than what is printed on the tire.

What was the accurately measured distance? How long was the ride? Have you tested the accuracy of the cyclometer? How did you test the accuracy of the cyclometer? What distance do you get if you drive the route in a car?

“A bit short.” Not sure what kind of unit “a bit” is, or how long “short” is. I ran a (measured and verified numerous times since it’s a race course) 6.20 mile route three times last week, and the 945 measured 6.21, 6.22, and 6.21 miles respectively.

I’ve had any number of similar results on different courses and have found the 945 quite accurate. Your mileage may vary. 

I am playing around on the sensor settings in F945 ... set it up to auto and same wheel size as the cyclometer .. will update on the diff.

The only thing I can assume is GPS from F945 and Cyclo are different. Not sure which one need to be updated or corrected. I will try to test again including Strava via iphone and compare the 3