Many people think that the larger the sub the better the sound, but what you need to do is measure the interior space of your car to determine the optimal harmonics. If the longest distance is, say, 8', you should have a speaker and box tuned to that size. Otherwise the sound wave curve might be too long for your interior. This is especially important in ported/bandpass boxes. In my Escape, I have 2 8" subs tuned to 10' wave and it's plenty of bass. For my Mustang, since it's a convertible, wave length had less impact, so I had to do things differently and built a sub box into a rear-seat delete. 2 6" JL Audio. A tuned box will get more out of an 8" sub than an un-tuned 12".