Size
The best subwoofer for cars, trucks, and other vehicles vary in size from 8 to 15 inches. Typically, larger subwoofers have louder bass, but that's not always the case. A variety of other factors also affect performance, including the sensitivity rating, enclosure type, and power handling. An 8-inch subwoofer is usually better than factory, particularly if it's set up for optimal performance.
Enclosure Type
This is an important feature because it affects the subwoofer's quality of sound. How the sub is enclosed can also impact performance. The best 8-inch subwoofer for a sealed box, for example, will generate the deepest base, while a ported enclosure and bandpass enclosure will permit higher levels of volume.
Frequency Range
A subwoofer is limited to the frequency range it can attain. A subwoofer with a smaller lower limit will produce deeper, lower frequencies. If you're looking for really deep bass, choose a sub with the lowest frequency limit. A high-power 8-inch subwoofer will typically have a lower frequency rating between 20 and 30 hertz.
Single vs. Dual Voice Coils
Single voice coils are useful if you want a quick and simple upgrade to your audio system. Dual voice coils give you flexibility when it comes to adding additional sound outputs. If you enjoy customizing your stereo to find the best sound, consider dual voice coils.
Power
Every subwoofer has a wattage rating that demonstrates how much power it can handle. It should be noted that this rating is for peak performance. A subwoofer that operates at its highest rating for an extended period of time will eventually overheat and fail. Look at the root mean square (RMS) to see how much power the sub can consistently handle.
Sensitivity Rating
To determine how powerful a subwoofer is, you also need to look at the sensitivity value as well as the power rating. A sub with higher sensitivity is able to produce the same level of sound with a smaller amount of power compared to a sub with a lower sensitivity. 
