To get gear ratios on Chevy trucks you have to look on the dash glove compartment and look for a decal that says: SERVICE PARTS IDENTIFICATION look at the picture below for a sample.
In this example the gear ratio for this vehicle is GU6 if you look at the chart below with all chevy RPO codes
CHEVY RPO CODES.pdf
I looked it the chart and it says is a GU6 AXLE,REAR,3.42 RATIO