car amplifier system

You weren't very specific, so I'll try to clear up a little here... speakers come in all flavors and shapes and sizes these days. Main speakers (in the front) are often 5.25 inches, but if you can fit larger ones you'll get more sound... the larger the speaker, the more air it moves, and thus the louder it will sound. Rear speakers in cars are sometimes larger because there's more room for them.
car amplifier

any update on this thread.