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Discussion Starter · #1 · (Edited)
Need a little help. I have a gm 10bolt 8.5" ,,ring gear. the rear is from an old nova 70's. 54 1/2" to 55" between backing plates. Its a little hard to get my tape measure through the shocks and springs. I have planned on putting in an Eaton posi unit with new axles.
I'm pretty sure that the unit now has 26 splines and I plan to change to 30 splines. not sure what gear ratio would be best. this is in my 31 chevy coupe with a sbc about 400hp through a supert10 4 speed. Since I'm not going to race this but drive it I am considering 3:70's or 4:10 gears. I'm wondering what other people have used or other ideas. thanks sc
 

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Beautiful little car. Usually its best to match the axle ratio to the first gear in your trans (3.73s with the 2.64 1st gear; 3.23s with the 2.88 1st for example); but in such a light car, I would say it really depends on what you want to do. 3.55 is a pretty good all around rear gear; which will work well with most ST 10s.
 

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Your 8.5 has 28 spline axles. If you want to upgrade it to 30 spline axles then get the 30 spline Eaton Posi and a set of Moser 30 spline axles. Moser is the only company that makes direct fit axles for this swap. All of the others will sell you a set of 12 bolt axles that have to be modified to fit the 8.5. The Nova 8.5 axles will be 29 5/8 inches long. You need to check an axle before you order anything because I have seen several of the old Novas that had 2nd gen Camaro rear ends stuck in them.

If you have the ST10 with the 3.42 first gear then you will not want to use a 3.73 or a 4.10 gear. A 3.42 or 3.23 would be better for that transmission. You need to find out what transmission you have before deciding on a gear.
 

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Discussion Starter · #4 ·
Your 8.5 has 28 spline axles. If you want to upgrade it to 30 spline axles then get the 30 spline Eaton Posi and a set of Moser 30 spline axles. Moser is the only company that makes direct fit axles for this swap. All of the others will sell you a set of 12 bolt axles that have to be modified to fit the 8.5. The Nova 8.5 axles will be 29 5/8 inches long. You need to check an axle before you order anything because I have seen several of the old Novas that had 2nd gen Camaro rear ends stuck in them.

If you have the ST10 with the 3.42 first gear then you will not want to use a 3.73 or a 4.10 gear. A 3.42 or 3.23 would be better for that transmission. You need to find out what transmission you have before deciding on a gear.
thanks i will look at the numbers on the t10 and see if I can tell what the gear ratio is. It should be on the web somewhere. sc
 

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Discussion Starter · #5 ·
rear end gears

after crawling under my car with a magnifying lite and trying to read the vin number with bifocals the best I can figure is the trany came out of a pontiac 1978, or 77, norwood, oh. the chart i found on line says it has a first gear 2:43 or 2:64 the numbers on the trany are hard to read. so what would be the best rearend gears? sc
 

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It's hard to say without knowing more about you and the car, but you can play with this some and get an idea of what you want the cruise RPMs to be with different gear ratios and tire sizes. Calculators
The 3.73 or 4.10 might be too much if you want to get on the highway much. They would be great if you never get more than about 10 miles from home.
 

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trans gear ratio

jack it up, shift it into first gear, chalk mark the drive shaft, and the front pully, Pull the spark plugs then turn the engine over with a wrench and count turns to get the drive shaft one rotation, , do the math.
 
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