77 grand prix SJ axle ratio
 

What rear axle ratio would a 77 SJ 400 ci typically come with?

Best way to know for certain is pull the rear cover & look at the inscribed numbers.

My Dad special ordered '76 & 77 SJ's, both near identical, Silver exterior/carmine dash & door panels with white bucket seat interiors/AC, tilt, pwr windows, pwr locks, cruise. Both were 400 cars.
The '76 had 2.93 gearing, the '77 had a 3.23 STT. Figured out the '77 gearing shortly before Dad sold the 77 SJ & received the '79 403 OldsmoBird, he'd ordered for my mom. For some reason, Dad thought the '76 had more get up & go than the '77, but his idea of performance was out on the highway, when he stomped on the pedal to pass another vehicle.

2.73 was also a popular ratio in the '77 GP's. If the '77 GP was built with a 301 4bbl, trying to remember but believe it received 2.41's. In the 90's through early 00's, I pulled the covers on a ton of '73-77 8.5 GP rears in Pick-N-Pulls & crusher yards. Pulling bolt in axles, posi's, & select ratio used 8.5 gear sets. Odd enough, have pulled 3.23 gears & posi's as well as tach & gauges & clocks out of a half dozen '77 GP's, only one set of tach & guages was from an SJ. The rest were out of LJ's.

From what I read in the AMA specifications, any 77 GP with the 400 had 2 possible axle ratios. The vast majority had the tire shredding 2.41:1. I did a 4000 RPM neutral drop ONE time - I got a minor "chirp" out of the tires - but it did have posi! (This was my college ride.)
The cars with the trailer tow package (Medium Trailer Group) got a much more responsive 3.23:1.
The white 77 GP SJ I drove did have the gauges, tach and a clock mounted under the radio, in front of the shifter. I would have kept the car but it didn't have the options I wanted - namely power windows or - trailer tow! It did have a steel sunroof, and when we sold the car (in family from 86 to 92), the sunroof still worked, never leaked and the clock still worked!
It was one of the best used cars my family ever owned.
The guy we sold it to got a few DUIs, possibly some wrecks - and the car was scrapped in 93.

Damn thats too bad. I lucked into a very nice 77. Unfortunately it dropped a valve 200 miles into my ownership, so I have a spare block at the machine shop now. I'll pull the cover when I have it on my hoist and verify. I was hoping for 3.08's or 3.23's. Did these come with the 8.5 inch 10 bolt?

stock rear for 73-77 was a 8.5 inch 10 bolt. Most were highway gears @ 2.41 ratio. Very few 3.08 or 3.23 were made.

I think the most common was 2.56. According to the 1975 Pontiac service manual the rear was supposed to be the MP 8 7/8”. This is the rear with the scalloped 10 bolt cover similar to the 8.2” but has a 12 bolt carrier. I have owned two 75 GPs. Both had 2.56 open. 1 was a late year build and had the 8 1/2” 10 bolt with 9 1/2” drums. My present one was an early build originality titled in November of 74 and it has the MP 8 7/8” rear with 11” drums. It has been upgraded to a 3.23 Safe T Track. Both LJ models with the standard 400 4bbl.