Speaker Size

[lfrancone]

How is a factory installed speaker size (diameter) measured?
Do you measure the exterior of the speaker or the opening that the speaker is mounted in.
I have a Chevy Silverado extend a cab and from what I can determine the factory speakers measure 6.75" O.D. but the opening in the door is 5.5" I.D.
What size speakers do I need the replace them with?