There are different types of gain control:
- Peak normalization will search for the loudest part of the file, amplify it to the loudest level possible and then amplify the rest accordingly. It's a pretty "stupid" technology in the sense of not making use of any psychoacoustics.
- ReplayGain is an algorithm that analyzes the files on a psychoacoustic basis and amplify them accordingly, so they all have the same perceived loudness.
I assume you are on Windows, but this works for OS X as well:
What does it do?
MP3Gain analyzes and adjusts mp3 files so that they have the same volume.
MP3Gain does not just do peak normalization, as many normalizers do. Instead, it does some statistical analysis to determine how loud the file actually sounds to the human ear.
It will analyze your files and make them as loud as possible, thereby equalizing all of them in terms of loudness. So normally, it should be enough to let this run on your library. MP3Gain includes a batch option for your convenience. They will not be re-encoded during this process and therefore not lose quality.
However, your car audio would need to support it. Try it out and let me know if it's not working.