Look for the following when buying a subwoofer for your car. The following specs should be made available from the manufacturer of the subwoofer. PLEASE DO NOT use this guide to determine what ampliphier you need to power your sub. When buying an amp, you usually want to match the RMS rating of the amp with the RMS rating of the subwoofer. PLEASE DO NOT use this guide to determine what enclosure type or what size subwoofer you will need. Refer to the manufacture of the sub to determine enclosure dimensions.

Qts- For a ported enclosure system, you want this to be around 35 or lower. For a sealed enclosure system, you would want this to be 40 or higher.
Qms- The higher the better. This gives a tighter suspension. Tighter is better.
Qes- The lower the better. You want a low Qms and a high Qes. Numbers can vary, so you may want to look at other subs to get an idea of what is high versus low.
Fs- In the majority of cases, this should be around 30Hz or lower.
Xmax- In most cases, the higher the better. This is the linear one-way travel of the cone.
Db- In most cases, the higher the better. DO NOT use this as an acurate way of telling a subwoofers output capability and should not be used as a comparison to other subwoofers to determine which one is going to be louder.

Follow this guide when looking for a subwoofer for your car, and you should have one banging system when used with the correct amp and enclosure.