If you don't have Nero then I'd recommend Burrrn (www.burrrn.net) as an excellent Audio CD burner. In fact even with Nero I'd still use burrrn.
You just add the mp3 files & it'll convert as part of the burn process.
Bear in mind that some car stereo systems will not play CD-R. The manual might help there. However, if you use quality media ( I'd recommend Verbatim Pastels & Super AZO) with either a CD writer or LG/Benq DVD writer then you should have success. I've said LG/Benq as these work fo rme , whereas my older NEC 1100 & 2500 wouldn't do it.
I've found that media quality is more important than burn speed but many would suggest burning at no more than 24 speed.