How can I tell the rear end ratio (308, 336, 355, 390, 411) on a '72 I'll be looking at later this week? It's a small block automatic but non-numbers matching. Are there numbers stamped somewhere on the outside of the pumpkin? If so, where exactly and how do I interpret them? I am concerned because the owner claims that the car is taching above 3,000 RPM at 70 mph. Thanks very much for your pointers!

 

It sounds like it's a 3.70 ratio.
On the '72 Corvette IRS rear suspension, the differential code is stamped on the bottom of the differential carrier housing just forward of the rearend cover. The axle code reads from left to right. The two-letter prefix designates the gear ratio and any other specific information about the axle. The third digit is the plant code. The next three numbers designate the actual day of the year this axle was built (001-365). The seventh digit represents the shift the axle was manufactured on. The last character letter is the Posi-traction source. If a space or no letter follows the shift code, the axle did not originally come equipped with a Posi-traction axle.

Plant Codes:* B = Buick* C = Buffalo* G = Detroit Gear & Axle* K = McKinnon Industries* M = Pontiac of Canada* O = Oldsmobile* P = Pontiac* W = Warren, MI

Posi-Traction Codes:* (-) = No Posi-traction* D = Dana Posi-traction differential* E = Eaton Posi-traction differential* W = Warner Motive Posi-traction carrier

Example: AA G 218 1 E* AA = Axle Prefix: 3.55:1 Corvette Posi-traction axle* G = Assembly Plant: Detroit Gear & Axle* 218 = Day of Year: August 6, 1972* 1 = Shift: First* E = Posi-traction Source: Eaton


’72 Corvette Rear-Axle Codes
Gear Ratio Code
Posi 3.55:1 AA
Posi 3.70:1 AB
Posi 4.11:1 AC
Posi 3.36:1 AX
Posi 3.36:1 LR

 

Here is a link to a web site taht has a ratio calculator

http://www.ringpinion.com/Calc_RPM.aspx

David

 

Thank you both very much!
Would '71s and 70's also have the same alphanumericals? I ask since I'm actively searching for a car in those 3 model years.
I want a 70-72 small block, 4-speed if at all possible, A/C, Coupe, Blue, Red or Silver, within 500 miles of Denver.

 

Chock the front wheels. Raise the rear tires off the pavement, put the trans in neutral, release the parking brake. Put a chalk mark on the driveshaft and the right rear tire. Count how many revolutions the driveshaft makes for one revolution of the tire. Should be close enough to tell which gear ratio it has. Do not try to determine rear ratio by the speedo. Perhaps, like my Vette, the plastic driven speedo gear in the trans tailhousing was incorrect causing the speedo/odometer to register ~30 mph faster than it was actually going.

BTW, there is no 3.90 vette rear. It is 3.70:1.

 

Looking at the codes on the diff will tell you what the car came with, but that is no guarantee that those gears are still in the car. Ya really have to find some way to get the rear tires off the ground and count the driveshaft revolutions, like 69 Chevy says, to be absolutely sure.


Scott

 

[QUOTE=MelWff;1578624412]It sounds like it's a 3.70 ratio.
QUOTE]



I just had my Vette out this weekend and the tach showed 3k at 70 mph. I have 3.70 gears with P235-60-15 tires.

 

Thank you gents, for all of the good information--I take note of it. Additionally, I stand corrected re 3.70 vs. 3.90 ratios. You are, of course, right.

 

Make sure you physically check it, don't go by what the tag says. I went by the tag which matched the tank sticker saying it was a 3.70 and after I threw a rod - found out that it was re-geared to a 4.11!

 

actually there was a 3:90 R&P, GM #3970551 available "over the counter"

 

there is torque converter lockup, clutch slippage, speedo errors, tach errors, tirewear.. all will mess up your calculation. plus there are aftermarket gear sets that are 'other ratios' than what were available from the factory.
with a 40 year old car, you never know what is in the pumpkin.

you can get in the ballpark with some of the above descriptions, but the only way to tell for sure is to count teeth.

 

Again, thank you all who've answered. I feel confident I have the information needed to keep me out of trouble.