Hello I will help you with your question,
If you can tell me all the codes that start with "G" from the label inside the glove compartment I can tell you what axle the truck left the factory with.
Let me know and I will help you with this
That is the VIN and it will not tell the ratio absent a VIS report.
There is a tag in the glove compartment with the RPO codes. Post all the three letter/number codes that start with "G" and I can tell you the axle ratios.
The G80 indicates a locking differential and the GT4 means this is a 3.73:1 axle ratio
Let me know what I can do to help you
To change the axle ratio and the tire size will require reprogramming the PCM and the ABS modules with a Tech 2 Scantool and a larger tire will give you better fuel mileage not worse so I suspect the drop in mileage you see is due to an aggressive tread and more rolling resistance of the new tires.
You would spend more to change the front and rear gears and in reprogramming than any gains in fuel mileage you would find.
Let me know what further questions you have now that I have provided your axle ratio.