The Stovebolt Site Forums The Parking Lot Big Bolts GMC 4000 top speed

I dunno about it screaming at 45, seems a bit slow for a 7.2:1 final drive to me. However it wouldn;t be the 1st time I've been acused of being a [censored]. :P
Just saying, my old dump, in stock configuration, with a 10/8.5:1 2-speed rear, & same size tires, will do 50-55 all day long.

By my calculator, taking the tire diameter and R&P ratio given, if your 5th gear is 1:1 you should be running around 48 MPH at 3000 RPM, and if you have the 0.82:1 ratio you should be running 59 MPH at the same RPM. There are around 9 sets of gearing for the NP540, but from what I have found 5th gear is one of those 2 ratios.
With that information, the question then becomes what's the RPM limit on your engine, or, at what RPM does the torque drop off. Given that number & I can tell you approximately what your top speed will be in each gear.