Ohms

Most cars use 4ohm, standard fare for car audio. I know some manufacturers, though, that use 8ohm, 6 ohm or even 10ohm speakers.

If you want to know for sure and you're up for the challenge, just take one of the speakers out and look at the back of the magnet, the impedance should be stamped on the back.
 
Stereos don't "put out" ohms. Ohms measure the resistance of the speaker. Most auto amplifiers, including the little one in the stereo, work with a 4 ohm load. Higher quality amps can handle 2 ohm or lower loads, putting out more power when the resistance drops.
 
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