4.30 gears were not availible on the Avalanche are you sure you don’t mean 3.73 gears or 3.42 gears? If your Avalance is a 2WD model you should get about 15 MPG city and 20 highway. For a 4WD it’s 14/19. 17.5 MPG is right on the money for normal mixed driving. Your tires are likely grossly overinflated.
There’s likely nothing wrong with your truck. 16 MPG is perfectly acceptable mileage for this vehicle. It’s a full-sized V8 truck, what kind of mileage were you expecting to get?