When they first came out with the mp3 format, they claimed that 128 kbit/s is as good as the sound quality of a CD and that any higher bit rate would only add unneseccery size to the mp3 file, saying that they cut out the parts of soundwaves that human ears cannot pick up, so 128 kbit/s is pretty much the essence of what you'd only hear even in 320 kbit/s mp3 file.

So pretty much every mp3 file I have is 128 kbit/s. But nowadays when I'm encoding things I use either lossless format or at least 320 kbit/s of mp3. I find it a bit trickier to play lossless formats like FLAC (Free Lossless Audio Codec) in a car stereo as they only play either traditional CD tracks or MP3 files like in the X5. If I really wanted to I would have to convert FLAC into a format that the car stereo can read, but then there'd be some loss during that conversion process...