How do I adjust the speedometer?

~28 to ~32s.. that is a big jump.

If your truck has a gear driven cable, then you can look into getting a new gear with different count teeth. I think you probably have a digital sender, but you'd have to confirm that. If digital, you can do some regearing (expensive) as well as get yourself a scan gauge and go through the calibration process detailed out in that link to the other thread (drive a physically known distance, mile markers, etc.. and measure the distance the ODO says and then set the offset in the scan gauge accordingly). Or live it with it. Really depends on how badly you want to know exactly how fast you're going.