Allen, I agree with Grigg...do the swap and don't look back. You will not regret it.
Here are a few measurements from when I did mine that you might find helpful. Not scientific, but the measurements were done comparing my speedo at 50 MPH to my GPS. Before any swapping, my speedo was dead accurate at 50 MPH according to my GPS.
When I changed the 4.56 gears over to 4.10's, I picked up 10% which is consistent with the math. When the speedo read 50, the GPS showed I was going 55.
Then I switched out the factory 17.5" rims with STA bias ply tires to a set of factory 19.5" rims with Bridgestone 8R19.5 radials. This gave me another increase of 10% over the 4.10 results. The compounded increase was 21%. After the two modifications, the truck was moving at 60.5 MPH with the speedo reading 50.
You can use this info to get a feel for what the swap will do.
Matt
