Technical 55 2nd GMC Rear-end Identification Help

Alright guys, a couple of questions for you. I have owned my 55 2nd GMC now for quite some time now. However, I have always kept the opinion that it just doesn't have the gearing to drive on the highway, so I drive around town and at max hit about 50mph. Anything faster than that and it is screaming in my opinion, but I will say a Tach was never hooked up to it.

So now about two weeks ago, the curiosity bug got the best of me and I started wondering what it would take to be able to go out and cruise on the freeway at 65/70. Shouldn't be too much to ask, but what I am having some issues with is figuring out my rear end.

So I hooked up a Sun Pro Tach to the truck, everything seems to work correct on it, and the truck reaches roughly 5k around 50 mph.

Then I jacked up the rear of the truck, spun the wheel to see how many times the drive shaft spins (Checked to make sure it was not posi which it was not) and the driveshaft spins almost 2 times per wheel rotation.

Ok to put all that information together, I did some research on the truck originally and found this, http://www.1955gmc.com/1955-59gmcguide.html, which states that my truck should have the "GM SS H033 Hypoid" rear axle which is the same from the Chevy trucks at the time. Further research turned up this page, http://chevy.oldcarmanualproject.com/chevyresto/56139.htm, which states that based on my rear end test I most likely have a 3.90 rear end. Cool that makes sense as the truck is pretty much 100% stock, minus a rebuilt transmission.

But where I run into some problems and have some questions is, trying to figure out what rear end I can switch to I ran into this site, http://www.richmondgear.com/2.html, which says that based on what my tach is telling me and my rear-end wheels ( 235/75-15R) My rear-end would have to be 8.0 or higher, that doesn't seem right. And then the other calculator says at 50 with my rear end I should be just under 3k on the tach. That isn't close for sure, the engine is screaming.

So what am I missing? My truck has the Pontiac 316 V8, original, and a 4 Speed Hydramatic that was rebuilt just about a year ago and that works really great. So I am not sure what I could be missing, any help would be greatly appreciated. Ultimately, my goal is to be able to cruise at 65/70 with as little modification as possible to keep the truck as close to original as possible. Thanks for your help guys.

 

Sounds like your tach may not be set on the correct number of cylinders? I can't imagine it is doing 5k rpm's at 50 mph, but could be wrong. I bet it is closer to the 3k (or maybe 2500ish if the tach is set to read a 4-cyl) which may sound like screaming, especially on a Pontiac V8.

 

'54 GMC hydro pickup, 248 & 3.90 axle with 245 x 15" rear tire ... 68 - 70 MPH @ 4000 rpm. Some big rear tires would be the easy way in your case, and that Pontiac will pull 265 - 275s easily.

 

Yes, larger rear tires are 'the poor man's overdrive'. I use 215 85R 16 's on my '49 Chevy 1/2 ton with a 3.90 rear end. I do have overdrive but I go over a local mountain pass in direct drive @ 3k rpms going about 65 mph. Larger tires will get you that speed.

 

Chevys had the 3.90 rear gears, GMCs had the 3.91 (or 3.92?) gears.

First figure out what rearend you have. Chevy rearend is the H033 it has a round, removable rear cover, AND a drop out center section.

The GMC should have a Spicer/Dana 45 which has a removable rear cover, and you can't take the pumpkin out. The cover is shaped kind of like a stop sign.

oh, and fix your tach. Might be one of those new ones that doesn't work with points.

 

I am in the camp that says your tach is incorrect...and actual rpm is probably high 2000's to 3K.

Ray

 

Wow, I feel like a complete idiot, you are correct, the tach was actually set to 4 Cyl, I checked it several times I thought before installing it, but as it turns out, not correct, so now it is reading correct which is in a lot more acceptable RPM range, I thought 1500 seemed high for idle.

Yes and no, I have done quite a bit of research and talked with several people including a Chevy GMC truck dealer associate at that time. in the 55 2nd GMC trucks the rearends are the same as Chevys, the Hypoid. In 56 the Spicer was an option but not standard and in 57 the Spicer became standard. - See Here http://www.1955gmc.com/1955-59gmcguide.html

So I guess my next question is going to be, is this the best option for lowering the gears? http://www.classicparts.com/1955-62-Rear-End-Gears-338-Ratio/productinfo/93-513/#.V04-jBUrLao or is the better places to go? These are a 3.38 gearing which I think will get me to where I want cruising at 65.

Also another dumb question, what is the difference between a GM 10 bolt rearend and the H033 or as I have found it referred to as recently the GM55T rearend?

 

There are several flavors of GM 10 bolt rearends, but mostly they have a cover on the back, and non-removable center section. Like the Spicer.

Thanks for the info on the 55/56 change on the rears, I didn't know that..not many GMCs to look at! Also the 1959 Positraction option on the Chevy trucks got you a Spicer rearend. I have one of those.

I don't have any experience with those 3.38 gears, but they may do what you want. I'd check out the condition of the rest of the rear end first, before spending the money on gears, and also see if I could find another type of rear end to use instead of dumping money in the old one. Depends on the long range plans for the truck, of course.