What amp alternator comes stock on my van (06 2500 express, passenger)
 

So I want to replace the serpentine belt but they are different sizes for the different alternators. I guess the two sizes they list are 105 and 145amps. I've looked up my VIN but different sites are saying different things. I've tried to see if my alternator says anything on it and I couldn't find anything useful. I've got a 2006 Chevy Express 2500 Passenger with rear AC. My VIN is 1GAGG25V861227006

What is your engine size? The VIN Decoder site compnine.com will give you a detailed and very accurate list of your van's build, it will tell you what size alternator you have. They charge $5 to read 5 vin's, highly recommended.

I have a 4.8 liter V8. I did check the comp nine it said something different than another site I'm curious which I should trust

I would trust compnine, free decoders missed a bunch of stuff on my van. If you're going to buy the belt locally, just buy both before you start, return the one you don't use.

Does Compnine give the RPO codes? If so, you can double check it with the RPO sticker on the passenger front door.

The decoder here(above, under tools) says you have a 105 amp.

Yeah unfortunately that's what I've seen, compnine says 145, others say 105, yeesh

Yes compnine shows the RPO code, for instance, mine is K68 GENERATOR,105 AMPERE 105 AMP

You mentioned your van has rear A/C, according to the decoder here it says "Alternator, 105 amps (Not available with (C69) Air Conditioning, rear.)
That's telling me you have the 145 amp.

Another thing, pretty sure you have 2 belts, the serpentine and a separate one for the A/C compressor.

Compnine didn't have my VIN.