The cd player in my car won't play cd's written above 16x or something, so I have to write them at the slower speeds. Depending on the cd-writer, half speed or less. There's info about this online if you're interested.
My gamecube would only read dvd-r's that were written at 4x speed. When I tried writing at 2x or 1x then it couldn't read them.
It's pretty random.
