1979-corvette Rear End Gear Ratios

[KEN STERLING]

CAN SOMEONE TELL ME-IF THERE WAS A STANDARD REAR END USED IN 1979
OR WERE THERE OPTIONS ON THE GEAR RATIOS-AND HOW TO ID WHAT YOU HAVE--MY TACH READS APPROX. 4500 R.P.M.. @ 80 m.p.h. --i have a turbo 350
transmission.---any help would be appreciated--thanxz-ken

 

[72Mako]

Ken, In 1979 I'm showing the following available. One of these codes will be stamped on your rear diff:

Code: OM 3:36 positraction
OH 3:55 positraction
OJ 3:70 positraction

Charles

 

[cwerve74]

You didn't memtion your tire size but assuming 225/70/R15 tires your tach & speed numbers would indicate a rear end ratio in the 4.59 range. Not one of the standards for that year. You have lots of RPM for 80 MPH.

 

[Glensgages]

I have one source-book listing 3.36:1 ratio with 4-speeds, and 3.55:1 used with automatics.
I'm not sure how accurate it is, but have been told it is correct for other years' ratios...

 

[Vettehead Mikey]

You didn't memtion your tire size but assuming 225/70/R15 tires your tach & speed numbers would indicate a rear end ratio in the 4.59 range. Not one of the standards for that year. You have lots of RPM for 80 MPH.

Tire size is irrelevant if the poster is referring to indicated MPH on the speedo and not actual measured speed on the road. The car could be on jack stands with no wheels on it (don't try this at home kids) and the speedo would still register 4500 rpm with 80 mph on the speedo.

It does sound like somebody put an extremely tall gear set in the car tho.

 

[cwerve74]

Tire size is irrelevant if the poster is referring to indicated MPH on the speedo


Yes that is true. Tire size does not matter if your assume as I did that actual speed is the same as speedo measured speed. Could have wrong gear in Speedo giving the combination of 80 MPH & 4500 RPM. Good point Vettehead Mikey. I am still learning. I guess a 1 mile test run at an indicated 60 MPH equal to 60 second would be in-order to verrify indicated MPH are real.

 

[KEN STERLING]

thanxz-

Ken, In 1979 I'm showing the following available. One of these codes will be stamped on your rear diff:

Code: OM 3:36 positraction
OH 3:55 positraction
OJ 3:70 positraction

Charles

i'll jack it up and check it out -the car does seem wound tight
but the speedometer might be off 10 mph -i just went thru a construction zone where they had a radar sign that tells how fast your going and it read 60 as oppposed to my car speedo reading 50. the car is new to me so i'll have to give it a closer look.

 

[cwerve74]

I don't think that jacking up it and running it is a good idea. I think the other post was making the point that the only informafion we have is MPH to RPM ratio. That does not allow someone to know the real speed. The gear in the speedometer and the tire size are the last key to that question. If you think the real speed is high 20%, then the rear end ratio might be in the 3.8 range or be the OJ 3:70 positraction rear end. You should find the OJ on the bottom of the rear end. If the speedo is off by that much you might think about changing the speedo gear at the transmision. A 20 minute job once you figure out what color/ratio you need.

 

[KEN STERLING]

I don't think that jacking up it and running it is a good idea. I think the other post was making the point that the only informafion we have is MPH to RPM ratio. That does not allow someone to know the real speed. The gear in the speedometer and the tire size are the last key to that question. If you think the real speed is high 20%, then the rear end ratio might be in the 3.8 range or be the OJ 3:70 positraction rear end. You should find the OJ on the bottom of the rear end. If the speedo is off by that much you might think about changing the speedo gear at the transmision. A 20 minute job once you figure out what color/ratio you need.

- ha ha---i 'm only going to check the
stamping for my gear ratio. that wouldn't even be funny -to get ran over by your own car-thank you for the help-corvette action center is the greatest.

 

[citruspilot]

You know my 79 does the same thing. That is its wound up pretty tight at 70-80 MPH. Now, in my case, the tach is always at about 3500 RPM because it doesnt work. Needs the tach circuit board. However if I stand on it the tach will finally catch up with the real engine RPM's like "above" 3500 RPM's. So as the original poster mentioned I too must have a pretty tall gear in my auto tranny 79 Vette too. I mean its really putting out the RPM's at 70-80.......like close to 4,000 RPMs my guess. And my car is stock off the factory line equipped.......no rear end changes here!

I was told by someone that in 79 there was a gas crisis and remember the speed limit was set back then at 55 MPH. At 55 MPH my car is pretty perky and revs about what it should at 55 MPH. I would guess about 2,500/2,600 RPM's. But, nowadays, who in their right mind drives 55 MPH??

Jeff

 

[Full_Blown]

Ken, In 1979 I'm showing the following available. One of these codes will be stamped on your rear diff:

Code: OM 3:36 positraction
OH 3:55 positraction
OJ 3:70 positraction

Charles

Hmmm? I was going to swap my existing gears for a set I picked up with a bunch of other parts. Problem is I just used the calculation of dividing the number of teeth on the ring gear by the number on the pinion and came up with 4:11. Too rich for my use. I also understand you would need a larger carrier to handle these gears.

The problem I have now is I don't know what I have. I have the carrier out so I flipped it over, the numbers on the bottom read OH 173 there's a couple of other numbers further to the right one is either a 6 or a 9 the other two are E-1. Any ideas? The original car was a 1979 L48 with a Turbo 350 auto. Any help is appreciated.

Steve

 

[cwerve74]

I would read this as OH for the 3.55 ratio the 9 for 1979 and the E-1 for Eaton Positraction. Not sure what the 173 represents.

 

[curtis]

quoting from Catalogue of Corvette ID Numbers: The rear axle number identifies the gear ratio, the build monthe and day, plant where it was assembled and the work shiftit was assembled on. exampleOMW2131 equals 336posi, Warren plant, 213day of year, first shift. here is a list of codes for 79:

OM..............3.36posi
OH...............3.55posi
OJ.................3.70 posi

plant:
Buffalo.................C
Chevrolet Gear and Axle......G
GM Canada...........K
Warren...............W

Shift:
1st shift..........1
day shift..........D
night shift.........N

Can't find an E anywhere in rearend identification. Maybe Its a K.

 

[citruspilot]

Hey Ken, it seems you and I have the same problem. I would hesitate going on a Corvette cruise because at 70 MPH my 79's engine would probably burn up!

I drove my car to the Charlotte AutoFair at Lowes Motor Speedway last weekend four days in a row. I had it on display. On the last day while departing the track I was getting on the highway and had about three other Corvettes getting on behind me. I speed up to about 65 MPH, which was the posted speed limit, and those other three vettes, all C-3's, blew right past me with normal sounding RPM's and just left me in the dust. I never could catch them. It was embarrasing I tell ya! :cry I could'nt even keep up out of fear of blowing my engine up........

Oh well.............. :eyerole

Jeff

 

[Glensgages]

I was getting on the highway and had about three other Corvettes getting on behind me. I speed up to about 65 MPH, which was the posted speed limit, and those other three vettes, all C-3's, blew right past me with normal sounding RPM's and just left me in the dust. I never could catch them. It was embarrasing I tell ya! :cry I could'nt even keep up out of fear of blowing my engine up........

Yeah, they'd pull you on top-end, but unless they're making a ton more HP than you (big-block, a hopped-up C4, or a typical C5), you'r depper gears would CREAM 'EM from a standing start or at the strip...