What makes a speaker specifically a car speaker?

Now for your replacement. Seems you are cost conscious (a smart thing). Start with what you are going to use the speaker for. If it is for a car, you will need auto type parts. The reason is because the temperature swings of outdoor equipment can cause moisture to condensate on your driver material (start cold, use it, it heats up, and you've got water!). So outdoor/auto grade will use moisture resistant or waterproof material. Auto speakers will also see a lot of sudden forces (drove over any railroad tracks, potholes, bad driveways, etc.?). So the material and suspension will need to be beefed up. Doing this means the driver coils will need more force, so more power handling. If you don't need to operate in a car, don't get auto drivers -- for the same price, you can get much better performing indoor drivers.

Once you know the type of driver you want, determine what you are replacing. Is it a general purpose 10" driver (wide response), or is it a woofer (bass), or is it a sub woofer (ultra low bass)? If you don't know check for electrical components in line on the wires going to the driver. If there are components you can calculate the rough frequency of the cutoff. Assume there is one capacitor that is 100uF, and your speaker is rated 8 ohms. Then the frequency going to your driver is (1/(8 x 100 x 10^-6) = 1250Hz), so your driver is a subwoofer and resonates above 1300Hz. Get another driver that has good response at low end and resonates above 1300. If you have other drivers in the speaker enclosure or if you have crossover components, you must match the advertised impedance (4ohm, 8ohm) with what your old driver is as you can see these values are used with the other components to build the response of the speaker. If there aren't other drivers or crossover components in the speaker housing your driver is likely to be a wide range. In this case you can replace it with any wide range driver -- even one of different impendance. Just remember to replace the driver in each pair with the same impedance -- and remember to check if your amp can drive the impedance you are connecting. A lower impedance driver will take more current.

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