Speedometer issues

It's common for the stepper motors in these gauge panels to fail, sometimes they completely fail, other times they just start acting strange. In your case you are probably not the victim of that problem. 5 mph is within the tolerance of "correct speed" that most vehicle makers use. That is because even the correct factory sized tires vary in actual diameter from brand to brand and even within a brand if there is a different tread design Then as the tread wears it also effects the actual speed. Using a scan tool that can talk to the OBD I computer on that truck you can feed in different speed signals and see where the needle sets at those speeds. That would be the only way to verify that you actually have a problem or not and you could correct it by moving the needle or adjusting the tire size in the software if it allows it, most vehicles from that time period don't. However as soon as you put on new tires it would be off again.

The normal tolerance is + - 5 mph with OEM tires from the factory for most of the automakers. Newer vehicles are a bit better, but there is still a built in fudge factor to allow for the build variances and tire changes. With current tire companies there are quite a few size differences in the "same" sized tires.
It is one of the reasons why many police vehicles use the same tire brands, sizes and tread designs. Makes it easier to keep those "certified" speedometers correct, even though they are actually only certified correct at 55 mph and many departments don't really check them the way they are supposed to.