Welcome to how audio works.
What is ‘real’? Have you not noticed that the same sound, is different when you move location. Your guitar played outside, sounds different from in your bathroom, and different from in your car? Acoustics works quite strangely. Outside, the sound from your guitar gets captured by your ears with no reflections of any kind, assuming you are in a field! Enclosed within a room or other space, you get reflections at some frequencies, and worse, maybe big chunks of sound absorbed by things in the space. In a cathedral, every strum of your guitar bounces off all the hard surfaces and one guitar arrives at your ears in maybe hundreds of directions, separated in time, by distance. Studio monitors try hard to put equal amounts of energy into sound at all frequencies. Car speakers might emphasise the bass and the high end and not bother with the stuff in the middle. Rap might sound great and a symphony orchestra might sound rubbish!
The answer to your question is that you need experience and perhaps different speaker systems that mimic your car’s system. That way you can assess better what you need to do to get better car mixes. BUT then your mixes will sound awful on others. A studio that specialises in certain styles may well have a system tuned to support that style, but most are tuned to sound the most accurate, so you really know what is in your music. Then you modify that for people in cars. Lots of my music is made for playing in theatres. My studio does not sound big, and does not have huge sub bass speakers, so my mixes have to be a bit weird to make them sound good in a venue. Most times I adjust correctly, but sometimes I still fail.
