When I first got my car, this wasn't an issue. The Bluetooth volume on my iPhone stayed at 100% and the volume knob on the steering wheel adjusted the stereo volume accordingly, but there was a Tesla software update a while back that synchronized the car stereo volume with that of the phone. In other words, if the car stereo was set to 50% volume, the phone's Bluetooth volume would also be set to 50%, so I'm really getting 50% of 50%, or 25%. The result is low audio output until you get to nearly 100% when all of a sudden the volume steps become huge. There's also an annoying lag between volume knob movement and loudness. Is there any way to force iOS to output Bluetooth audio at 100% all the time?