Differences between total distances between a Fenix 7 watch and an Edge 830 bike computer.

Did you have a look at the pace data of both? Maybe there's something odd there that might provide an answer.

You ask question without provide info like satellites etc

So, to clarify the GPS situation; the watch had all 3 satellite systems "ON" plus the multiple frequencies... The Edge 830 had both the GPS and Galileo systems "ON" but does not offer the multiple frequency option... Now after several more testing runs including a short run (less than 3 kilometres) and two long runs (one was 44 kilometres the other one 50 kilometres) using the car's onboard Garmin which I installed back in 2018 (can't remember the model), the car's trip computer with both the watch and Edge 830 set on the car's dash to get a good satellite reception, the results were very, very close… All 3 Garmins and the car’s trip computer were within 0.1 kilometre of each other. So now back to riding the mountain bike on the mountain, through the forest and so on… The results were very disappointing to me… On very short runs through the woods (about 4 kilometres long), the difference between the watch and the Edge was 0.7 of a kilometre, the watch always having the higher of the two readings… On longer runs (10 kilometres), the difference was 1.5 kilometres, again the watch reading higher… I have to conclude that somehow the GPS signal is not as good in the woods thus the difference but you would think for the price you pay both Garmin units should be very close in their data even when we ride in the forest… I may just get a wheel speed sensor installed on the from wheel to see which one of the two units is more accurate.

Did you try comparing the tracks to see if there are obvious differences that could explain the  the difference in the distance?

Garmin Edge 830 has been released 4 years ago and has an older GNSS chipset, so I think Fenix 7 is probably more accurate, especially that you use it with all satellite systems and multi-frequency GPS. That gives it a better ability to correct multipath GPS errors. 

If you have a speed sensor, the Edge will be right.

The different satellite's setup could be an issue.

From my experience with 820 and I guess 830 is no further than 820, in the woods the GPS signal is lost in dense areas.

Etrex 35 touch instead have super good signal in dense woods.

If i ride with eTrex and Edge, the edge will report shorter distance.

I guess Fenix have greater accuracy than Edge because is multiGNSS.

Speed sensor is the key for accurate distance.. try to connect it to both devices and see the differences. GPS are not that accurate and when riding on wooded trails with so many turns, you will no get good results using any type of GPS.

So, to clarify the GPS situation; the watch had all 3 satellite systems "ON" plus the multiple frequencies... The Edge 830 had both the GPS and Galileo systems "ON" but does not offer the multiple frequency option... Now after several more testing runs including a short run (less than 3 kilometres) and two long runs (one was 44 kilometres the other one 50 kilometres) using the car's onboard Garmin which I installed back in 2018 (can't remember the model), the car's trip computer with both the watch and Edge 830 set on the car's dash to get a good satellite reception, the results were very, very close… All 3 Garmins and the car’s trip computer were within 0.1 kilometre of each other. So now back to riding the mountain bike on the mountain, through the forest and so on… The results were very disappointing to me… On very short runs through the woods (about 4 kilometres long), the difference between the watch and the Edge was 0.7 of a kilometre, the watch always having the higher of the two readings… On longer runs (10 kilometres), the difference was 1.5 kilometres, again the watch reading higher… I have to conclude that somehow the GPS signal is not as good in the woods thus the difference but you would think for the price you pay both Garmin units should be very close in their data even when we ride in the forest… I may just get a wheel speed sensor installed on the from wheel to see which one of the two units is more accurate.

Update... After getting some answer from the Edge forum it seems a wheel speed sensor is required to get accurate read outs, no stand alone GPS or Watch on its own can provide accurate data riding through the woods especially on tight windings trails. So I just ordered a wheel speed sensor which I will receive later today and test out.

Yes. i always use speed sensor with my bike and is 100% accurate.

recent ride using speed sensor with my edge 1040 and my Fenix 7