Edge 840 reads 25% faster speed than my Forerunner 255 and Wahoo Elemnt?

if you have speed sensor in the wheel, set the correct wheel value, or check if Element is set correctly. one is not for sure

No wheel speed sensor. The Forerunner and Elemnt both agree very closely.  The Edge is clearly too fast as the speeds sadly do not align with my capabilities.  

That's an odd one.  Are you using mph or kph? 

Does the GPS track for both look the same? 

Are the distances the same?

Mph

Yes and yes 

The gps track is also clean, no weird teleportation. 

Doing some more experiments, I see that during a ride the distance and live speed on the head unit matches, but the elapsed time on the Edge is much lower.  Thus when uploaded the speed is wrong.  

Are you doing lots of stops and starts, or cycling very slowly? If you have AutpPause on then your Edge will.pause the timer at, for instance, traffic lights. You'll cover the same distance but in less elapsed time once you remove stopped time from the total.

Try switching off AutoPause to see if that helps.

Nope, dirt roads so almost no stops.  I did a quick around the block test where I started and stopped the devices at the same time.  The Edge gave me 15:39 and 18.0mph vs 19:28 and 14.5mph on the Forerunner and 19:36 14.4mph on the Elemnt.  

It does look like auto-pause is on, hence the shorter time, which in turn will give a greater speed.

Auto pause is on for every device.  But this happens at a steady speed without any stops.  I can ride around the block steady at just under 15mph and the Edge tells me 18mph, which is certainly faster than I was going.