I know the only positive way to tell is pop a cover, but that being said, does anyone know what gears they put in most 1980 cj5 with gm 151, sr4, dana 300 combo. My jeep has 31/10.50/15 on it and not sure yet how it even does with them until i get my new exhaust manifold.
If you want to have a stab at it in 2 minutes crawl under the back end and look at that AMC20. to the left of the diff cover you should find one or letters stamped on at the factory. I have given the letters above for the non-locking diffs.
The 31s are only slightly larger than stock which were about 29". From memory that is 7% difference in circumference, which you will not notice. Whilst a low powered unit it is a light vehicle, if not blinged with winches and heavy bumpers and wheels.
The problem is that the 2.73 were already a poor choice on stock wheels.
AMC axle codes are stamped on the differential casting between the cover and the tube weld hole on the passenger side) beginning in 1970 for the AMC 20 series rearend, 1972 for the AMC 15 series. First four letters/ratios are 1970 rear axle codes stamped on the differential housing cover flange at about 3 o'clock:
A = 3.54:1 B = 3.15:1 C = 2.87:1 D = 3.91:1 E = 3.58 (12/43) F = 3.08 (13/40) G = 3.31 (13/43) H = 2.73 (15/41) J = 2.37 (19/45)
The 15 series had 10 bolt oval cover with drain plug center right.
15 series rearend
The AMC 15 series rearend has a 7-9/16" ring gear. It was used in 1965 and earlier models with 196 ci or 199 ci inline 6 engines. The small cars (cars other than those listed below as "big car") used the AMC 15 rearend with inline 6 engines, AMC 20 rearend was used with V8 engines. The AMC 20 rearend could be ordered in fleet cars with inline 6 engines.
20 series rearend
The big cars (Ambassador, Classic, Rebel, Matador and Marlin) used the AMC 20 rearend with 232 ci inline 6 and bigger engines.
The series 20 was used in Jeeps as well as AMC passenger cars. A Jeep 20 series rearend would have a 5 on 5" bolt pattern. Passenger cars used the 5 on 4-1/2" wheel bolt pattern.
Given that your Jeep is 33 years old and also that CJ owners tent to tinker/upgrade/swap more than most, it's probably about a 50/50 chance that you still have the original gears. Before making a decision about a possible gear change, I'd open the case and verify what's in there (it's a good idea to do a periodic inspection anyway).
For sure.. I plan on opening up both for inspection and change fluid. PO broke a bolt on the rear cover and it has been leaking fluid past his patch work, aka gasket maker glob,.. Got to get all that taken care of as well... Thanks for all the info. Just got the exhaust manifold in today. if i can get a windshield for it ,and all the fluids to stay where they belong, I can give her a road test.