If you have an iPhone do this:
Turn on Hotspot.
Connect the car to the hotspot. It isn't always so easy with the encryption choices but I think you leave it off and then it'll ask for the password.
Have Spotify up and running in the car with a song you want to play, a longer one is better for precision. Note the length but don't press play just yet.
Go to the cellular menu and look at hotspot usage at the bottom of that screen, there will be a selection for that. The biggest problem will be if the number is in GB and not MB. You don't get any precision if it is in GB.
Play the song on Spotify
Check the usage again and subtract the original number to get usage for that song.
Divide that by the song length in seconds.
That'll give you a ballpark usage number. It could be more or less, depending on how much other stuff the car is using the wifi for, but figure it'll either be around 300 or around 100 if the low bitrate spotify plays. The problem with this method is that this will be the bitrate over Wifi, and the LTE bitrate could be different. With Tidal that is certainly the case, but we have no way of measuring the LTE bitrate without some kind of monitor hack.
If you have an Android phone there is likely some similar method. I just don't have one to check.