How do 1992 peterbilt 379 speedometer/odometers function

I recently purchased my 2nd truck a 1992 peterbilt 379. Few minor things left to do before my first employee starts driving the Pete 2 Mondays from now. The pyro and odometer aren't working right. I pulled out the pyro bought a new one and that issue is fixed. It still functioned but the internals and face we're dislodged in the gauge so I decided to replace it and make it right as I know how important this gauge is. I may send the old one out to get repaired and keep on the shelf. The speedo odometer combo part number Q43-6010-005 they no longer make. The speedometer is way off and reads about 45 when traveling 70 mph. I'm going to replace the gauge because of the odometer not working and I'm hoping this will fix my speedo issue as well. Probably not that lucky..The number superceded to 49911RT or 49911. There are a few companies that make these. Amatek does make an analog odometer version. After doing some reading I now know there are at least 3 different types of ways speedos measure speed. Electronic transmission control box is one, electronic sensor somewhere is another and there is even another type as well. Do you guys know which type they used in a 1992 peterbilt? Which type you recommend I replace the bad one with? And how I can fix it if its not just the gauge I'm going to replace? There's a lot here and I truly appreciate any and all input and advice. Thank you!

 

And how does the odometer function that came in my 1992? I feel it's important for me to learn these things so I can stay on top of them.

 

I installed the new speedometer today and the new one is off the same amount as the old one which needed to be replaced anyway because the odometer was broken. Read about 40 mph while driving 60 mph. There's a what looks like new sending unit/sensor in the box in the truck from the previous owner. My buddy told me if the speedo is reading 40 mph then the sender is working. I matched the 10 switches on the new one to the old one. Next step is to use the instructions and calibrate it properly. My question is if I do that and it's still way off are there any other components than the sensor / sending unit that screws into the back of the Tranny?

 

I'm going to for sure. I'm wondering if the speedo is reading something even though it's off if that can be anything other than the calibration? Do you guys know how the sending sensors work?