[u]ReplayGain[/u] sets all of your tracks to a common “loudness”.
Assuming your car stereo doesn’t directly support ReplayGain, [u]MP3Gain[/u] changes the gain of the MP3 itself so it works with any player. There are also similar AACgain and WaveGain versions.
There is a [u]Replay Gain Plug-in for Audacity[/u].
iTunes has something similar called Sound Check that works on iTunes, iPods, and iPhones (and I assume the iPad).
NOTE - Many quiet-sounding songs are normalized (maximized). You cannot make these quiet songs as loud as your loud-sounding songs without clipping (distortion) or without using dynamic compression.
This means that the louder songs have to be reduced and ReplayGain (and the similar programs) must use a slightly lower target volume in order to have “room to work”. Many of the louder songs in your library will be quieter after applying ReplayGain.