What I would do (and this is kind of advanced, but I do recording as a little hobby of mine) is download Audacity and load up the songs that sound crappy into that and look at the 'wave form'. Songs ripped/recorded can vary in 'fidelity' and you can tell by looking at the size of the wave form. For example, here is a wave form representation of a song I recorded with Audio Hijack Pro from a YouTube video:
The wave form is quite low, and I'd have to crank up my stereo whether that would be on my iPod or on a CD that I've burned. I fix that by amplifying it with the built-in amplification plug-in before burning to CD or importing into itunes. (BTW, it's a live performance by Dire Straits of 'Sultans of Swing'.)
Here is an internet radio stream recorded also with Audio Hijack Pro from Quicktime:
As you can see, that sucker is LOUD with a capital L. I'm not sure what settings in iTunes one can set, but a little processing of the audio prior to burning can make a difference. I know that most of the songs I've ripped from CDs are usually of the lower volume while things I've recorded end up being more like the larger wave form you see above and I kind of just live with it. But learning about how sound is represented digitally can help you even out your volume settings so perhaps they won't sound quite so bad in your car.
Welcome to the world of audio processing.