BT Music mode streaming
Highway does not increase the amount of data consumed by the music service. The data consumed depends upon the audio quality setting for the music service.
For Spotify, normal stream quality is 96Kbps. This is equivalent to 42Mbytes of data per hour, or 24 hours of listening per GB of data allowance. The extreme audio setting (320Kbps) uses 140Mbytes per hour and allows 7 hours of listening per GB.
Spotify mode started from Highway/Pure Go
When you playback offline Spotify content using Pure Go or Pure Highway to start playback, Spotify will always treat the playback as online.
This means regardless of the content being stored offline on your device, Spotify will choose to stream the content consuming data.