The answer is 2-way: yes and no.
If you burn music from MP3, you'll get the same amount of time on a disc, as if it were wave files. If you burn MP3 as audio, the files are converted to regular audio (wave doesn't need this conversion) and get as big as wave files again.
When burning a data-cd you can put 650mb of data on a standard disc. If you burn MP3s (those are about 10x smaller than wave is) you can fit 10 times as much songs on the disc.
If you want to play MP3 data discs in your car, you need a CD player that can play MP3 cdroms. These cd players are getting more and more common, but are still a little more expensive (they cost about ?300) than regular CD players..