Car Subwoofers, or Subs, is where the fun starts, but again you may need different type of subwoofer drivers depending on SPL vs SQ. You will also have to keep in mind the air space required per subwoofer, watts, RMS and peak, and size. Subwoofers also come in more than one shape now. You have round, square and triangle subs to choose from. Subwoofers also come in 2, 4 and 8 ohms with both single and dual voice coils. Other things to know and understand when buying a subwoofer is listed below. You can also read the car audio blog to get my reviews of all different types of car audio equipment including car subwoofer reviews.
Car Subwoofer Features and Information, Lets Review

RMS Power Handling: This is the amount of continuous power, in watts, that the subwoofer can handle. Make sure you match the subs power handling to your car amplifiers power.

Peak Power: This is the peak amount of power, in watts the subwoofer can handle. Remember, this does dont mean you can send the subwoofer the amount of power for a short time it will happen will normall music in short burst.

Frequency Range: Frequency range gives you an idea of how low a sub can play. Keep in mind, though, that the actual performance of the sub can depend on a lot of variables, like how the subwoofer is mounted.

Sensitivity: Sensitivity goes along with power to achieve high output. A subwoofer that has a higher sensitivity rating requires less power to produce the same amount of sound as a model with a lower rating.

Impedance: Ohm of the subwoofer. This will be very important for choosing becuase it will affect how the system has to be wired. most subwoofers will come in either 2, 4, or 8 ohms.

Woofer Material: The type of material the subwoofer is made out of will affect the sound quality. Poly / Plastic: Probably the most used material for car speakers, they are weather-resistant and have good internal damping. Composite: Mica, CMMD, Glass-imides, HCL, and IMPP are all types of composite materials used for speakers cones. These materials are stiff and strong. They all can producee great sound with excellent dispersion.

Surround Material: Santroprene: This is a popular new durable plastic material. Urethane: Urethane is a plastic compound with proven flexibility and weather resistance and is an advance on natural rubber.

Top Mounting Depth: This will give you the hieght of the speaker, and will help you determen the size box you will need for it or if this subwoofer will fit in your car or truck.

Mounting Diameter / Length: Simply the width of hole the subwoofer will need to slide into.

Number of voice coils: A dual voice coil subwoofer has two separate voice coils. This offers more flexibility in wiring than a single voice coil sub. The DVC sub can be wired in three different ways: parallel, series, or independent. This way, you can wire the sub according to your system design and preferences.

Subwoofer Enclosure: Type of box your subwoofer will be mounted in. The main types are, sealed, vented (ported), and bandpass.