Something ain't right - new gears in rear end

[rgunlock]

I've got a 78 K15 with (as far as I know) the original SM465, NP205 and 3.08 gears. I went up to 33" tires during my build, so gears weren't quite right to handle various situations - right turns, etc.

About 7 years ago, I had some axles rebuilt professionally with 4.10 gears and limited slip differentials for another project that got put off. I also have an NV4500 and my plan is to swap these axles and NV4500 into my '78. This past weekend I started by swapping out the rear axle.

Pulled the 3.08 axle and transferred my fully rebuilt rear brakes intact - still mounted on backing plate - to the 4.10 axles. Installed new axle shafts into the 4.10 axle. Filled with limited slip additive and gear oil. Cover on and installed the axle onto the truck.

Now here's the interesting part/problem. I knew that I'd be running at higher RPMs because I haven't swapped out the sm465 yet, but I'm hitting the higher RPMs at lower speeds than the gear calculators suggest. I'm running 2500 RPM at 48 MPH in 4th gear. The gear calculators say that I should be doing 60 MPH at 2500 RPM. I tested my tach against the tach on my timing light and they matched. I'm using a GPS for a speedo so speed should be accurate. According to the calculator, my gears would be 5.12 in order to be doing 48 at 2500.

I suppose the drivetrain shop could have installed the wrong gears but before I jump to that conclusion I'm asking for suggestions on other possibilities I should check. Thanks --Rick