Why should you upgrade the speakers in your car and why:
There are many reasons why you should consider changing the speakers in your car. If it is an older car, and you do not have tint, the speakers probably took a lot of UV damage from the sun and has deteriorated. Older speakers are also single cone speakers, made out of paper and are not coaxial. Coaxial speakers will have an extra tweeter or midrange on top of the main woofer to give you more highs (voices, and instruments)
Speakers come in different sizes, and come in standard sizes. They start from 3.5 inch on up.
Below is an example of 3.5 inch speakers:
The most common speaker sizes in cars are 5.25 speakers (smaller)
and 6.5 inch speakers
and 6×9 speakers which are in the rear of cars
Most Fords will use 6×8 speakers. 5×7 speakers and 6×8 speakers are interchangeable. Do not mistake 6×8 speakers for 6×9 speakers.
Speakers can also come larger than 8 inches. These are still car speakers, but typically are called subwoofers or car subwoofers. They range from 8 inches to 12 to as big as 15 inches. The larger the subwoofer, the deeper the bass.
Sometimes the subwoofers come in boxes. Which means you do not have to make your own box for it, and it is tuned to the subwoofer. This makes things a lot easier
Speaker adapters are available for some cars to make them fit. For example, a 6.75 adapter to make 6.5 speakers fit in a 6.5 opening.