Was trying to get some ammo (research) together so when I take my truck to the dealer next Friday to convince them gm must regear the diffs for the 35.5 inch tires they sold me with my truck I found this..
A major drawback to upping the tire size is getting power from your engine to the ground. Installing different-sized tires from stock will effectively change your truck's gear ratio. With a bigger tire, the truck will feel like it is geared higher (numerically lower), which is great for highway cruising, but not for low-end grunt, off-the-line acceleration, or passing power.
To bring your truck back to stock performance, it is important to re-gear the truck accordingly. A simple calculation will tell you what gear ratio would get you back to your stock equivalent. The calculation is your new tire diameter, divided by your old tire diameter, multiplied by your old axle ratio, will equal your new axle ratio (new tire diameter/old tire diameter x current axle ratio = new axle ratio). For towing or performance, you would want the next gear ratio lower than the stock equivalent. So, if our '95 project Silverado had 30-inch tires stock, and a 3.73 axle ratio, that calculation would tell us we need a 4.103 (which rounds off to the readily available 4.10) gear ratio to get us back to stock. . And, keep in mind that 4x4s need both the front and rear diff re-geared to the same ratio

So In my scenario my stock tire size being a 245/70R17 or 30.5 inches diameter, and the tires size they sold me with the truck new are a 325/60R20 or 35.5 inches and 3.42gears 35.5/30.5=1.1639344x3.42=3.9806556 so I think they should at least give me 4.10 gearing to return the engine performance to the ground.. Im gonna take what I found here with me. They cant argue whats in Black and White..