Will jump this over here -
So the answer here kinda varies as a “48kbps” stream can vary quite considerably based on how much of that is actually audio.
You always lose a small amount to overheads, but from there it can depend a lot on how much is used for data - here’s a worse case scenario (plus a fun error) - over a third of the nominal rate is used on slideshow/data
There’s no real pattern here, other than the ABC and Community Radio being poor at it - SEN have been good with it in Melbourne, while Magic loses 20% of their 96kbps to data, but don’t even do a slideshow.
2CH currently have slideshow info from the last post I saw, so I wouldn’t be surprised if their rate is less than ideal - especially if they didn’t reconfigure much from data rates that would have made sense at 128kbps.