Help with gear transmission advice

[Frank M. Hardcastle II]

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.

[moody]

Post your Vin . Someone may be able to tell you what it was built with.

[schoitmike]

I just posted about the same thing but my new rig has an auto tranny, im afraid to drive it over 55mph because the engine is screaming. It is very likely we have 4:10 gearing, and 3500rpm seems real high for a diesel, most I know red line at 2800, these may be diffferent but I dont know.
your truck sounds like a truck I almost got, If you ever think of selling in the futre let me know.
Did your truck have an owners manual? there may be some engine data there, if so will you post it, Ive been searching the net for days and no tech info.
thanks and good luck,
mike.

[DmaxMaverick]

Both of you have direct (1:1, no OD) for high gear. Unless you are running really small tires, your rear gear ratio could be 4.56:1, especially if they are CUCV's. Check the rear gear ratio yourself, and don't trust any markings or RPO's you find. The gears may have been changed at some time.

If you have an open diff or loose limited slip, raise one wheel and rotate it exactly 1 rotation. Count the turns of the drive shaft (tranny out of gear or P, E-brake off). The driveshaft will turn exactly 1/2 the actual ratio. A 4.10 will a little more than 2 turns, and a 4.56 will turn 2-1/4 turns. If you have a locker or tight limited slip, you will have to raise both wheels (they will turn at the same rate, in the same direction), and the shaft turns will exactly match the ratio.

Another option is to remove the rear end cover and count the gear teeth on the ring an pinion. Divide the ring gear teeth by the pinion teeth for the ratio.

[HammerWerf]

Frank,

From you description, The tranny may be the SM-465. I have on in my 84 C20. Good tranny, generally sounds like it is chewing on rocks when in neutral and the clutch engaged. Chevy put that tranny in everything up to the C-60's. Visual from under truck, 2 PTO plates, and a total length of about 12 inches.

Given that you have a c-30, rear gear most likely will be a 4:10 . Like DmaxMav said, you could have a 4:56 back there, I think it was an option in those trucks. Rear axle is probably a Dana-70. Cover has 10 bolts and is a full floater.

HammerWerf

[thejunkman2005]

Ok, I might be wrong here. But chevy used the SM465 tranny for everything from big to small for years! The 1/2 ton trucks got them, and few C60s got them also. It's the grand daddy of all standard transmissions, and will take more than you can imagine to break it. It does not have OD, but otherwise has great gear ratios. Top of the line tranny right there.

Now, for rear gear ratio.. That's a little trickier.

Now, in the early years, chevy put a tag on the glovebox with the gear ratio, right there, rear simple to read, no prob. Just open the glove box, and look for a sticker with all the trucks options.

Now, in the later years chevy changed that tag over to codes that you can no longer decipher with ease.

If you have the second style tag you have another option for checking gear ratio.

Jack up one rear tire, mark the tire with soapstone on the bottom, then mark the driveshaft directly towards you. Turn the tire one complete turn counting how many times the driveshaft goes around. Then simply multiply that times 2, and you have your gear ratio.. Simple enough?
Standard
If the driveshaft goes around 1 and 3/4 times, you probably have 3.73s
if the driveshaft goes around 2 times, you have 4.10s
if the driveshaft goes around 2 and 1/4 times, you have 4.56s.

I do this all the time, as I'm trying to find a good set of 3.73s for my future dump truck which will be powered by a 6.2 diesel.

Most 1ton trucks will run 4.10s, or 4.56s. My buddies 1ton 3+3 dually 454 auto runs 3.73s! Which is pretty amazing for that big of a truck.

Good luck,
Len

[Frank M. Hardcastle II]

HammerWerf,
"From you description, The tranny may be the SM-465. I have on in my 84 C20. Good tranny, generally sounds like it is chewing on rocks when in neutral and the clutch engaged."

Thanks HammerWerf, now I know that it

[Frank M. Hardcastle II]

DmaxMaverick,
"Both of you have direct (1:1, no OD) for high gear. Unless you are running really small tires, your rear gear ratio could be 4.56:1, especially if they are CUCV's. "

What is a CUCV?

"Check the rear gear ratio yourself, and don't trust any markings or RPO's you find."

What is an RPO?

"If you have an open diff or loose limited slip, raise one wheel and rotate it exactly 1 rotation. Count the turns of the drive shaft (tranny out of gear or P, E-brake off). The driveshaft will turn exactly 1/2 the actual ratio. A 4.10 will a little more than 2 turns, and a 4.56 will turn 2-1/4 turns. If you have a locker or tight limited slip, you will have to raise both wheels (they will turn at the same rate, in the same direction), and the shaft turns will exactly match the ratio. Another option is to remove the rear end cover and count the gear teeth on the ring and pinion. Divide the ring gear teeth by the pinion teeth for the ratio."

Thanks, I have never had that explained before. I

[DmaxMaverick]

[quote=Frank M. Hardcastle II]DmaxMaverick,
"Both of you have direct (1:1, no OD) for high gear. Unless you are running really small tires, your rear gear ratio could be 4.56:1, especially if they are CUCV's. "

What is a CUCV?

"Check the rear gear ratio yourself, and don't trust any markings or RPO's you find."

What is an RPO?

"If you have an open diff or loose limited slip, raise one wheel and rotate it exactly 1 rotation. Count the turns of the drive shaft (tranny out of gear or P, E-brake off). The driveshaft will turn exactly 1/2 the actual ratio. A 4.10 will a little more than 2 turns, and a 4.56 will turn 2-1/4 turns. If you have a locker or tight limited slip, you will have to raise both wheels (they will turn at the same rate, in the same direction), and the shaft turns will exactly match the ratio. Another option is to remove the rear end cover and count the gear teeth on the ring and pinion. Divide the ring gear teeth by the pinion teeth for the ratio."

Thanks, I have never had that explained before. I

[Frank M. Hardcastle II]

schoitmike,
"I

[Frank M. Hardcastle II]

Post your Vin . Someone may be able to tell you what it was built with.

Moody,
"Post your Vin . Someone may be able to tell you what it was built with."

Great Idea. There seems to be a wealth of knowledge here that is absent at my local GMC dealer.

Thanks Moody.

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