I assume that your car radio requires the files on your SD to be in MP3 format. That's a 'compressed' audio format and it's possible to select different levels of compression when you 'rip' the files from CD. (A radio station, hoping to maintain the very best quality audio for broadcasting might choose to use a low level of compression, resulting in relatively big files. A university wishing to archive the lectures of some of its academic staff might choose to use a fairly high level of compression, as audio quality wouldn't be so important; they'd then end up with some very small files).
So there's no fixed answer to your question. However I've just examined some tracks that I've ripped from a CD using the default settings in Windows Media Player and they average out at around 1 MB per minute. So each gigabyte of memory would give you around 1000 minutes of recording time, meaning that the 64 GB card you mention would hold about 64,000 minutes, or over 1000 hours.