I am new to this board and have searched other posts for advice. I just purchased a 1984 GMC 3500 4dr. dually with 48,000 verified miles. It came with a huge camper and other electronic devices I'm unfamiliar with. I have removed the camper but have trouble with the transmission and gearing. I can't find anyone including the dealer to tell me anything about the transmission and rear end. It has a 6.2L with a Banks Turbo. The transmission is a 4 speed manual. It has a compound low in the upper left position with 1st gear in the lower left, 2nd upper right and 3rd lower right. I have been searching the internet and think it may be something referred to as a SM465. I have no way of confirming this as I can't find anyone to decipher the VIN. I have been told that the rear end will have a small tag of some sort attached to a bolt that will have some sort of encripted number identifying the gear ratio. I can't find it. While driving this truck back from South Dakota where I bought it I felt like I was going to blow up the engine at 65 MPH. The tachometer showed about 3200 RPM's. At 70 MPH it is about 3500 RPM's. I shift out of 1st into 2nd at 15 MPH. Out of 2nd into 3rd at 25 MPH. Can anyone help me with this delima. I have been told that if I change the rear gear ratio I will benifit by about 500 RPM's. I've also been told that I need to change the transmission to a NV4500 5 speed. these are not inexspensive modifications for me and will take some time. From what I read here my RPM's at cruising speed need to be near the 1800 mark.