Unfortunately, eggrider is the only solution at this time. I have two but dont like to use them as much due to the unreadable display and small/sticky buttons(the main reason).
In addition to wh/km, I also like total wh used. The two allow me to adjust my ride (speed/distance) on the fly to fully use the battery. If I am having a good day efficiency wise, I will ride longer and/or faster. Maybe I went too hard at the start and now need to tame it down for the rest of the ride.
These are both features on my Grin CA3 and I use them all the time.
My current 860C display only show volts and while I am getting used on how to interpret it, its less than ideal and requires mental math while riding.
I have the tools to write my own 860C firmware to do this (basically modify 860C TSDZ2 software for BBSxx) but havnt started the project yet.
https://cycles.eco/collections/tsdz...h-open-source-firmware?variant=32699435221074
https://cycles.eco/collections/tsdz...ource-firmware-updater?variant=29815890280530
I will put this on github when Im done and mention it here on EBR, dont expect anything soon.
Another option would be to modify this software
https://github.com/danielnilsson9/bbs-fw
Basically you could make the wh/mile calculation on the BBSxx itself and then return this to the display as voltage so your voltage reading would be your efficiency. Many other data item you could replace with efficiency depending on your display.