To me the 192kbps sounds best to my ears on a car stereo.You see 128kbps & 256 kbps also recomended I guess it's a matter of preference.Considering a comercial CD in the cdda format uses 1411kbps I think 192kbps is compressed enough.
You might get 3 more albums at 128kbps but I counted the songs one one of mine with 8 albums & it was 126 songs so how many of those do you need to carry in your car?Maybe 3 if your on vacation.I think one would be more than most of us would need for just in town driving.