The speakers in a 2006 GMC Sierra 1500 crew cab are 6.5 inches. However, the speaker manufacturer will refer to this size as 6.75 or 6 3/4 inch speakers.
4x6 are the size of the door speakers on 2004 GMC Sierra. Standard cab
The stock size of the speakers in a GMC Sierra are 4x6 in the rear. If they have not yet been switched out with an aftermarket, they will be the 4x6.
All four speakers are 4x6. I just ordered new 4x6's for my 93 GMC Sierra, and that is the stock size.
6.5" round door speakers
The 2001 GMC Sierra has 6 1/2 inch speakers in the front as well as in the rear.
The 2006 GMC Sierra-2500HD has a 6.0 L base engine size.
The 2006 GMC Sierra-1500 has a 4.3 L base engine size.
The 2006 GMC Sierra-2500HD has a V8 engine.
2006 E250 speaker size is 6x8"
usually 6.5 or 6.75 (same mounts, same speakers)
The 2006 GMC Sierra-1500 has a V6 engine.