I wouldn't spend a bunch of time and money on speedometer work. Just use a gps with a speedometer readout on the screen. Way more accurate than a mechanical speedo anyway, plus it tells you the speed limit and goes red when you go over. That's what I do in my truck.
I was going to suggest a gps speedometer like these:
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but they are still a little pricier than I thought. They have models for cars, motorcycles and boats, no reason you couldn't use one in a big truck.