TBH I've done some tests with comparing MP3 and at a decent bitrate I'd argue it's virtually indistinguishable to uncompressed music.
Obviously I use FLAC for my own music collection but MP3 is perfect for loading up either the memory stick in one car or the large classic IPod in the glovebox of my campervan. Even with the pretty nice system I've installed in the van (JBL component speakers, an amp and nice little underseat sub) I doubt you'd hear quality differences while travelling if there were any to hear.