ring and pinion question

[kmfb]

I picked up a 1970 GMC Sierra Grande short bed(cut down). The PO decided to redrill the front and rear end to the small 5-4 3/4 lug pattern for aftermarket disc brakes and his wheels he had. I picked up a complete rear and and complete front end (hub to hub) for $100 from a buddy of mine. I measured both 12 bolts to make sure they were the same width. After yanking both axels out of both rear end I looked at the ring and pinions. Both are 3.07 ratios, the rear end under the truck is 14:43 tooth count and the spare rear end is 13:40 tooth count. Why would GM use 2 seperate ring and pinion setups for the same ratio?

[TBONE1964]

Originally Posted by kmfb

I picked up a 1970 GMC Sierra Grande short bed(cut down). The PO decided to redrill the front and rear end to the small 5-4 3/4 lug pattern for aftermarket disc brakes and his wheels he had. I picked up a complete rear and and complete front end (hub to hub) for $100 from a buddy of mine. I measured both 12 bolts to make sure they were the same width. After yanking both axels out of both rear end I looked at the ring and pinions. Both are 3.07 ratios, the rear end under the truck is 14:43 tooth count and the spare rear end is 13:40 tooth count. Why would GM use 2 seperate ring and pinion setups for the same ratio?

Do both ring gears have date codes stamped in them? It shoud have the month and year. If it does nit, then it is probably not an original GM gear set.

My thought is one may be an aftermarket replacement? The 14:43 would be correct for a 12 bolt 3:07 gear in a truck rear end.

[kmfb]

The ring gear in the truck is stamped 3817779 14:43 GM 4-70 1. Im assuming 4-70 is april 1970. The ring gear in the spare rear end is stamped 14018118 13 40 GM 1 80 1.
The spare rear end stamping is hard to read since tooth cuts are half way into the stampings if that makes sense. I was told the spare rear end came out of a early 70's truck but im thinking its from a 1980?

[Mike C]

Is the ratio marked? The math suggests that the 43/14 = 3.0714 would round down to 3.07 and the 40/13 = 3.0769 which would round up to 3.08.

This is a great example of how the numbers can and do vary a little (4.10 vs 4.11 or even 4.09 depending on the axle manufacturer) and exactly how that works.

Don't know enough about 1/2 ton square bodies to know if 3.08 was the common three-oh gear or not.

Is this a leaf spring truck I assume? Are perches narrow on both for the 2.25" springs? An '80 model 12 bolt would have perches for 2.5" springs.

[kmfb]

the 80 rear end is a spare i bought off a buddy. He was told it was an early 70's 12 bolt but that proved to be wrong. Im not using anything from that rear end except the actual axle shafts.

[57taskforce]

I'm not sure if it helps or not but my 3100 is on an 88 k/5 frame and it's got the 43/14 set in it. The rear end is from a mid 70s half ton but the front is date coded to match the frame so I'm sure it's original. If all you need is the axles than it shouldn't make a difference. Honestly if one set was 3:07 and one was 3:08 it wouldn't make a big enough difference to worry about. Look at the Dana front 4:10 and Eaton rear 4:11 K/20s for example.