Can average speed be set to calculate from elapsed time?

If you turn off the auto-pause and don’t manually pause then the “time “ is elapsed time and the average speed is based on elapsed time, ideal for Audax where the clock is always running. There are some “Audax” CIQ fields which also help, see the excellent “Time in Hand” by Robert Sexton

https://apps.garmin.cn/apps/341a30bc-d557-4564-bf3f-cd5c8dae2f47

if you want to later review moving speed etc then most planning sites (like RidewithGPS) will calculate moving speed from your uploaded activity file

I have rode many Audax rides from 100's up to 1400's and from experience, their is absolutely no need to know your "Riding Time Average Speed", it relates in no way to how you are progressing or anything, it is a useless bit of information, its just a novelty. As already mentioned, turn off "Auto Pause" and then you will get Average Speed based on your elapsed time which is the most important. All you would do is "lose" a data field which could be used to display more useful information and on a multiday event, it just adds to the overall drain on the battery.

But, if you want to have both average speed based on Elapsed time and Riding time, use the HMS DIY data page creator in "Connect IQ and you can design a data page to display both. Most of my data pages are made from HMS DIY 1 -6 as there are 6 copies of it and you can install all 6 if need be. Also with "DIY" you can use the "Time to Dest" based on current speed and "Time to Dest" based on Avg speed, both will give different times and you can use both, also two different "ETA" based on current speed and avg speed, I only use "Time to Dest" based on current speed. Don't use the Garmin "Time to Dest" or "ETA to Dest" they are never correct, their useless, they can be a long way out and are only correct when you get very close to where you are going, they have an odd alogorythm based on average speed, plus a percentage maybe and as I can never fathom it out, it must also add a random number or it throws a dice and adds the score?

Connect IQ Store | Watch Faces and Apps | Garmin

Also as mentioned, use the “Time in Hand” by Robert Sexton, I use it all the time, it tells you all you need to know, how much "“Time in Hand” you have so you can plan how long you can dither in a control or have a long rest or sleep etc.

When riding, always ensure your current riding speed (not average speed based on riding time) and average speed based on elapsed is higher than the minimum for the event, usually around 7mph. So basically that means, that if you ride at 15mph, for every hour you ride, you will gain roughly an hours "Time in Hand" so after 10 hours riding, you should accumulate roughly 10 hours time in hand which is good going, but, you will lose some of that going through controls and stoppages etc so you may only have 7 hours or more.