got a stereo in my car that takes SD cards.

I have 3 kicking about the place a 2 gig, and 2 x 8 gig cards. One of the 8 gig cards is class 6 and in a camera, photos write to it much quicker than they do to the 2 gig card. The other 8 gig is nameless but apparently its a toshiba card?

Problem is when the 8 gig cards are pushed into the stereo, it takes about 45 seconds to come up with any tracks, (they are full)

Is it because they are full so I might be better off with less tracks on each or because they are slow cards to read from?

2 gig is a lot faster in the car.

It's probably because it is full. I have a 32Gb card in my car stereo, and my head unit ( Alpine ) splits the card into blocks of 1000 songs in order to get over the problem of slow start up due to having to read the entire card, even then it takes a minute or two to read in the 1000 tracks.

Of course the simple thing to do would be to take each 8Gb card, 1/4 fill it, and see how long it takes to start up the stereo.