I would also like to see this feature added to the Android Auto interface. After taping the Speed button a few times on the Android Auto interface, it becomes impossible to know what the playback speed is, without looking at the AntennaPod app on the phone.
Showing the playback speed incorporated into the icon, as suggested by @kacnje would be good. Or, if possible, an overlay message on the screen confirming the speed, that fades withing a few seconds. Or both!