Jim Carter wrote:

I used to believe the same thing about GMC and Chevy, only cosmetic
differences. But now after owning two GMC Sierras I am beginning to
suspect some other differences. Even though the use hasn't changed, the
GMCs require about 1/2 the maintenance as the Chevys did. Simple things
like sticky throttles, intermittent switches, loose exhaust brackets,
etc. don't seem to be plaguing our GMCs like they did the Chevy variant.

My wife says they "sound" different too; "tighter and more
put-together".

Just shows you the games our mind can play one us. They are made in the
same plant. http://www.autofieldguide.com/articles/070207.html

I believe they are actually made on the same production line, but the
same people, with just a few parts (grill, some interior parts, etc.)
being different, but I can't find any proof of that at the moment. The
engines and components are identical so any difference you are seeing is
a sample difference, not a Chevy vs. GMC difference.

Matt