89 2500 Transmission/Rear End Questions

[likearaptor]

Hi,

A couple of months ago I got my first truck. It's a 89 Chevy Silverado ext cab, 4x4, 350, 5 speed, with a flat bed. The engine was overhauled around 250,000 and a new/rebuilt transmission was put in it's place. I know from the pictures in the manual that this isn't the same 5 speed that came with the truck. Underneath all it says on the transmission is to make sure to refill it with a specific castrol oil. On the gear shifter knob it has a L-1-2-3-O-R. Does anyone know where I can find out what type of transmission I have?

I am not sure what ratio the rear end is. The codes in the glove box are all rubbed off so I can't see what it came with. Is there somewhere I can see the ratio?

The speedometer is off about 5 mph (so when I am going 55 it is really only 50 mph). I imagine that is because of the tires (The tires it is suppose to have is 245/75R16 but it has 235's). I did replace the speedometer gauge cluster and I heard that if you got the speedometer from a truck running a different ratio in the rear end it could be off, I never tested the old speedometer with a GPS before replacing it. In reality I just needed to fix the odometer since it was stuck so I could put in the old gauge set and just move the good odometer over. But I would like to know how I could get my speedometer working better