This might be a boring question but it seems important to me.
I am about to to do a 3 year tuneup/maintenance on my 1997 Blazer,
which includes changing the serpentine belt. I have a GATES Part # K060960
on there now. It's got about 40,000 miles on it. It just makes a little noise
at start up but goes away once it warms up. I am guessing it just needs to expand a little
along with everything else it is touching. I might buy the same one, its done fine.
A top of the line belt costs around 25-30 dollars. I have noticed that the more expensive
ones are all made out of EPDM Rubber. I have also noticed that they are all close in length
but different. Some say effective length 96 inches and some say 96.5 inches.
Since they all claim to be made out of the same high end EPDM rubber, I was wondering
why the different lengths. For those interested here is the definition of what EPDM Rubber
is. I copied it from Wikepedia.

EPDM rubber (ethylene propylene diene Monomer (M-class) rubber),[1][2] a type of synthetic rubber, is an elastomer which is characterized by a wide range of applications.The E refers to Ethylene, P to Propylene, D to diene and M refers to its classification in ASTM standard D-1418. The “M” class includes rubbers having a saturated chain of the polymethylene type. The diene(s) currently used in the manufacture of EPDM rubbers are DCPD (dicyclopentadiene), ENB (ethylidene norbornene) and VNB (vinyl norbornene).

The ethylene content is around 45% to 75%. The higher the ethylene content the higher the loading possibilities of the polymer, better mixing and extrusion. Peroxide curing these polymers give a higher crosslink density compared with their amorphous counterpart. The amorphous polymer are also excellent in processing. This is very much influenced by their molecular structure. The dienes, typically comprising between 2.5 wt% up to 12 wt% of the composition serve as crosslinks when curing with sulphur and resin, with peroxide cures the diene (or third monomer) functions as a coagent, which provide resistance to unwanted tackiness, creep or flow during end use.