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Greetings all; I'm changing out the ring and pinion from my factory JM stamped axle tube 2.73 and want to lower the ratio to 3.42. However, I've been told that my 10 bolt posi, that the carrier might not be able to take a 3.42 and that I might be limited to 3.08. Has anyone lower the gear ratio in a 10 bolt posi and if so, what ratio did you end up with?

These were the options available for the 74 which leads me to believe the 3.42 should be ok but, wanted to get your feedback on this.

1974

JA - 2.73 open
JB - 3.08 open
JC - 3.42 open
JM - 2.73 posi
JN - 3.08 posi
JP - 3.42 posi
 

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I went from 2:56 to 3:42 but when you have those tall gears the carrier usually will only go up to 3:08 I know this because I went to 3:08 blew my rear end gears then back to 2:56 then to 3:42 that's where the problem came into play the carrier won't go far enough over to the passenger side of the car to mesh properly you can get a ring gear spacer from summit and it comes with longer bolts I personally haven't had a problem with mine it wasn't a big job you may need to change the shims on your carrier possibly too sounds like a big job it's really not it can be a bit of screwing around setting up new gears you get easy ones and hard ones to set up
 

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I bought a 73 Nova SS new back in the day and it came with those POS 2.42 gears. Wouldn't bark a tire to save it's life but it would fly on top end. I took it to a speed shop in Colorado Springs and was able to have 4.10 gears installed. Ran those for a couple of years and then backed off to 3.73.

Mark
 

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For a 8.5" 10 bolt the carrier breaks are:
2 series- 2.41-2.56 ratios
3 series- 2.73-5.38 ratios

JB, that is really handy! I have never seen the defined ratio break line between 2 ans 3 series carriers. Thanks for sharing!

Miles
I bought a 73 Nova SS new back in the day and it came with those POS 2.42 gears. Wouldn't bark a tire to save it's life but it would fly on top end. I took it to a speed shop in Colorado Springs and was able to have 4.10 gears installed. Ran those for a couple of years and then backed off to 3.73.

Mark
Mfeller, I thinking 3.73s are the best choice to replace my 2.73s as well. How are the 3.73s for around town? How are they on the freeway? Thanks!
 

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IMO 3:42's are the best all around ratio for performance and highway. I ran 3:73's for a number of years and they were borderline rpm wise for highway cruising. The rpms were higher than I would have liked to have seen.
 

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What transmission are you running? If you are not running an overdrive trans and plan to run above 60 for any length of time, I'd recommend the 3.42's over the 3.73's since you are not planning to race it. The lower gears will be more fun light to light, but you lose top end. I went with 3.70's in a for 9 inch and until I installed my OD trans, I kept waiting for that last shift that never occurred. My mileage went to h e double toothpicks as well. Just food for thought. What we do to our cars is personal and for our own enjoyment. Just try to think about all the angles when making changes.
 

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My 66 wagon currently runs a mild 350 with 3.73's in a 10 bolt with 25.6" diameter tires & a 2004R OD trans, and I'm considering going to either 3.43's or 3.23's. The 25.6 diameter rear tires (225/60x15's) turn 813 revs per mile, and are about the biggest that will fit in the wagon's rear wheel wells (this a street car that is never tracked and sees quite a bit of long-distance freeway driving - I'd like to drop rpms a bit further for freeway running).

The formula for figuring engine speed for any given tire size and diff / trans ratio is tire revolutions per mile x differential gearing x transmission gear ratio = engine rpm. For simplicity, I generally calculate at 60 mph - here's how the tire size/gearing/rpm formula pencils out for my current set up, and for the ratio changes I'm considering....

Current setup - 3.73 diff ratio:
813 (tire revs per mile) x 3.73 (diff ratio) = 3032 (driveshaft rpm) x .67 (4th gear OD ratio) = 2031 rpm @ 60 mph

3.43 ratio:
813 x 3.43 = 2788 x .67 = 1868 rpm @ 60 mph

813 x 3.32 = 2625 x .67 = 1759 rpm @ 60 mph

Substitute your tire size (Tire Rack's "specs" section for each tire will give you the diameter and revs per mile for any size tire you want), diff ratio and top gear ratio and you can calculate engine speed at 60 mph for any combination you can come up with.

Bow Tie Overdrives has a good section on their web site on calculating rear wheel torque based on diff gearing and first gear ratios that's also useful for figuring out where you want to be on both ends of the gearing spectrum

Several years ago, I had a 65 El Camino with a similar set up to what's in the Nova Wagon (mild 350, 700R4, 3.31 gears and 235/70x15s) - it turned barely 2000 rpm at 75, and got 20 mpg at that speed.
 

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I went from the factory 2.73 to a 4.10 gear. Not a freeway flyer for sure, but oh what fun. Going the same route on the next one.
 

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My 66 wagon currently runs a mild 350 with 3.73's in a 10 bolt with 25.6" diameter tires & a 2004R OD trans, and I'm considering going to either 3.43's or 3.23's. The 25.6 diameter rear tires (225/60x15's) turn 813 revs per mile, and are about the biggest that will fit in the wagon's rear wheel wells (this a street car that is never tracked and sees quite a bit of long-distance freeway driving - I'd like to drop rpms a bit further for freeway running).

The formula for figuring engine speed for any given tire size and diff / trans ratio is tire revolutions per mile x differential gearing x transmission gear ratio = engine rpm. For simplicity, I generally calculate at 60 mph - here's how the tire size/gearing/rpm formula pencils out for my current set up, and for the ratio changes I'm considering....

Current setup - 3.73 diff ratio:
813 (tire revs per mile) x 3.73 (diff ratio) = 3032 (driveshaft rpm) x .67 (4th gear OD ratio) = 2031 rpm @ 60 mph

3.43 ratio:
813 x 3.43 = 2788 x .67 = 1868 rpm @ 60 mph

813 x 3.32 = 2625 x .67 = 1759 rpm @ 60 mph

Substitute your tire size (Tire Rack's "specs" section for each tire will give you the diameter and revs per mile for any size tire you want), diff ratio and top gear ratio and you can calculate engine speed at 60 mph for any combination you can come up with.

Bow Tie Overdrives has a good section on their web site on calculating rear wheel torque based on diff gearing and first gear ratios that's also useful for figuring out where you want to be on both ends of the gearing spectrum

Several years ago, I had a 65 El Camino with a similar set up to what's in the Nova Wagon (mild 350, 700R4, 3.31 gears and 235/70x15s) - it turned barely 2000 rpm at 75, and got 20 mpg at that speed.
 

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Hi Barry. Your post very helpful and right along the lines I am researching for a rod purchase. Would you be willing to provide your phone # to my email offlist so I can call you and we can talk about this further? Or I can provide my # in reply to your email.
67babycar
 

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Hi Barry, I am looking for a street rod and doing the research. Rear end ratio is part of that. What you have to say is spot on with that. Would you be willing to supply your phone number if a send my email or supply your email and I will send you my phone number?
Thanks 67babycar in VA
 

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For a 8.5" 10 bolt the carrier breaks are:
2 series- 2.41-2.56 ratios
3 series- 2.73-5.38 ratios

Miles
Hey Miles,

I am new to getting this involved, but we were gonna put a Lunch Box Locker in and wanted to change the gears. How do I tell if I have a 2 Series or 3 Series.
Is it safe to assume that if I have 273's then I have 3 series.

and I should be able to fit a set of 373's in it.

Secondly, how do I know how many splines my Axles have, originally a 250 3spd Auto.

Third , We now have a slighlty worked, moderatly Cammed 307, coupled with a Super T-10 with I think a final drive of 1.0
What would be your recommendation on gear ratio? or should we do it backwards using the formula based on the Cam Spec and power range?

thanks
 

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Hey Miles,

I am new to getting this involved, but we were gonna put a Lunch Box Locker in and wanted to change the gears. How do I tell if I have a 2 Series or 3 Series.
Is it safe to assume that if I have 273's then I have 3 series.

and I should be able to fit a set of 373's in it.

Secondly, how do I know how many splines my Axles have, originally a 250 3spd Auto.

Third , We now have a slighlty worked, moderatly Cammed 307, coupled with a Super T-10 with I think a final drive of 1.0
What would be your recommendation on gear ratio? or should we do it backwards using the formula based on the Cam Spec and power range?

thanks
Yes if you have an 8.5 w/2.73 you will have a 3 series carrier (unless it has a ring gear spacer). All the 8.5 diffs were 28 spline stock on the novas for 6 cyl and v8 alike. Yes I would get the rpm range of the cam, and tire size to help calculate ratio. I will say with a 305 bored .020" over with an rv cam I was running a 3.08 but that was with a th350 sitll a 1 to 1 final gear minus torque converter loss.

Miles
 
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