Size: You’ll want to first decide if you're going to simply replace the existing speakers — in which case you can just measure and match, then drop the new ones into the speaker receptacles. If you’re going to install a completely new speaker system, it will require a more complex installation.
Sensitivity: This rating lets you know how effective the speakers are at converting power (watts) into volume (decibels). Speakers with higher sensitivity, require less power. So if you’re working with a lower power receiver, you’ll prefer speakers with a high sensitivity, while speakers with low sensitivity work fine with high-powered receivers.
Power handling: This rating, also known as the RMS (continuous power) rating, refers to the level of power (watts) your speakers can handle. To get the most out of your system, you’ll want speakers that can handle the power your receiver is capable of putting out. Speakers in a low-powered system don't need to be able to handle lots of power and so can have a lower RMS rating, while a system with a powerful receiver requires speakers with power-handling that matches receiver output.
Types of Car Speakers
After-market car speakers recreate your music's vibrant highs and midranges. There are two types of car speakers, based on how the tweeters (responsible for highs) and woofers (for midranges) are housed:
Coaxial: Usually sold in pairs, coaxial speakers pack multiple components such as tweeters and woofers into each unit. They split the audio frequencies coming from your receiver multiple ways and direct them to each component. The number of ways the sound is split is a good indication of the clarity and detail your speakers will produce.
Component: At the higher end, component speakers are a separate unit for each tweeter and woofer, allowing you to place them in your car wherever you prefer for the best sound. Installing your tweeters near head level results in clearer highs.