Unless you compare the actual belts, it may seem like the new belt is the wrong size when you have it routed around the pulleys the wrong way. I had to find a diagram when I replaced the belt on my car.

Cut the old belt off the engine, and lay it down on a table, match the new one to it.

Yes to which ?

yes

I am needing to replace my serpentine belt. I have a VE Commodore Series 1 Omega 2009 With SIDI Engine. While searching Online I found at least 4 different belts to suit. I have tried 6pk2555 which is about 20cm to big, and 6pk2553 which I would believe to also be too big. I found that a 6pk2120 Fits but cannot find anywhere that this belt is meant to fit.

Could it be I need larger pulleys to account for the larger belt ? or should I stick with the 6pk2120 ?