I just bought a 85 cutlass Salon. It came with the window sticker that says it has 2.41 gears in the rear. I was wondering how to tell the difference between the 7.5 and the 8.5 rear axle (do you count the bolts 10 or 12) because I want to upgrade to the better rear axle and just want a bolt in. I would like posi so if If anyone knows what kind of car to look for and how to tell a posi from a non. Any help would be appreciated. The gear ratio is not all that important. They have wagons all the time at the scrap yards by my house.

 

I know I probably have the 7.5 axle. If I installed an after-market posi rear differential would that make the 7.5 axle stronger? I am not planning on building my engine to much as of now. I am hoping to run around 400-425 HP. Would my rear end with the after-market posi diff be able to handle that much HP?

 

Ok well there were many 10 bolt rears over the years.

What makes a rear differential strong are BIG PARTS usually. So the bigger the pinion the bigger the spider gears will deliver the power to the ring and through the carrier to the spiders and then to the side gears and out to the axles AGAIN BIGGER axles are better.

your 10 bolt 7.5 rear is a small diff compared to most 10 bolt diffs on the market. The 7.5 8.5 8.2 8.875 9" All pertain to the ring gear size. Anything in the 8" range is usually sufficient for the muscle car street applications.

The spider gears in most posi or open differential (non posi) are the very most weak link and have been improved over the years. 7.5 went to 7.6 and the improvements went from small spiders to larger spider gears and the axle shafts were also increased in size.

If you are street bound the best up grade you can get for your 7.5 is a 1990-2003 posi factory but better yet aftermarket Eaton and the aftermarket 28 spline axles with improve the rear 10 fold and I think you will be ok.

BUT if you need a GN 10 bolt 8.5 rear I have one for sale here for 1000.00 and comes with good used brakes. posi 3.73 all cleaned up and out checked out and reset for performance driving. Even has the original; parking brakes too.

 

Can you tell the difference between the 7.5 and the 8.5 by measuring the cover or the distance between bolts? I am wondering because I just painted my car and installed my Keystone Klasics. The rear rims and tires fit ok on my 83 but they seem to stick out farther on my 85.

 

It's a 7.5.

Only the 84 H/O and the 85-87 442s got the 8.5, and the only ratio was 3.73:1.

 

7.5 and 8.5 G-body rears were the same width.

All 1978 -1987 7.5 g-body rears again were the same width. Just your eyes fooling you OR there is damage to the rear or car.

Jim

 

But if I'm walking through the junk yard how can I tell them apart!!

 

Well if your in the junk yard use a 10 bolt TRUCK 1/2 rear as reference...That cover and cast center will be the same size as the 8.5 GN rear. 7.5 covers will be more of a square-ish cover compared to the 8.5 completely round cover.

I have posted pics for you. Jim

 

Thanks.

 

what vehicles came with the posi from 90 to 2003. and would i have to change to a 28 spline axle. also how can i tell what gears a car has because i think my old cutlass has 3something for gears and it would be a cheap swap for now.