'86 Caprice Sedan: 7.5 or 7.625 rear axle - Hot Rod Forum : Hotrodders Bulletin Board
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Old 10-23-2007, 06:37 PM
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'86 Caprice Sedan: 7.5 or 7.625 rear axle

I've spent a good deal of time searching this site and related links but haven't found a good solid answer to my question: What is the easiest way to tell which size ring gear is in my rear axle?
This car is my daily driver and tow vehicle for my dragbike and I want to change gear ratios in order to make towing easier on the drivetrain.
I've noticed that some gear mfgs. list a 7.5 gear as well as a 7.625, while others just list a 7.5/7.625.
Since I need to get proper id before I order parts, I'm willing to remove rear cover and even pull an axle, just can't tear down completely and then order the parts.
Thanks in advance for any light yall can shed on this.

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Old 10-24-2007, 01:05 AM
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7.5" will have 26 spline axles and 7.625" will have 28-spline axles.
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Old 10-24-2007, 05:57 AM
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Help w/picking a gear ratio

Thanks, Triaged.
This car is used for daily commuting thru town, slightly hilly, 25-50 mph speeds. Also tow a vee-front enclosed single axle trailer, gross weight 2500 lbs. 2 or 3 weekends a month anywhere from 45 to 75 miles one way on 2-lane highways and some Interstate highways.
Engine is stock Chevy 305, trans is B&M shift-kitted and cooler equip. 700R4.
I need recommendations on best ratio for rear axle for this application. Plan on using this car for a long time, and a swap to a 350 engine a couple years down the road is not out of the question no no real big power enhancement is planned.
Any and all comments are appreciated.
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Old 10-24-2007, 06:05 AM
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I would go with a 3.73, it would be perfect with the OD trans.
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Old 10-24-2007, 12:09 PM
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Thanks, 1ownerT. Would it be possible to tow in OD with 3.73 gearing at least on the flatter high-speed parts of the trips, say anything 60+ mph or will this put too much strain on the trans? If not, may go with 3.43 and tow in 3rd gear like I do now with the 2.56 gears, just hopefully not spend as much time in 2nd gear w/my foot on the floor.
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Old 10-24-2007, 12:26 PM
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You should never tow in OD with that trans.
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Old 10-24-2007, 04:49 PM
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Got that, Triaged.
Thanks to all who responded.
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Old 10-26-2007, 03:05 PM
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Quote:
Originally Posted by voodooracer
I've spent a good deal of time searching this site and related links but haven't found a good solid answer to my question: What is the easiest way to tell which size ring gear is in my rear axle?
This car is my daily driver and tow vehicle for my dragbike and I want to change gear ratios in order to make towing easier on the drivetrain.
I've noticed that some gear mfgs. list a 7.5 gear as well as a 7.625, while others just list a 7.5/7.625.
Since I need to get proper id before I order parts, I'm willing to remove rear cover and even pull an axle, just can't tear down completely and then order the parts.
Thanks in advance for any light yall can shed on this.
it being an 86 caprice it should have an 8.5 rearend.
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Old 10-28-2007, 04:54 PM
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And so it is. 8.5" geared 2:56. Thanks runn141.
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Old 10-28-2007, 07:59 PM
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A jump from a 2:56 to 3:73 is pretty big. A change to the 3:41 "might" be a little better depending on tire size.
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Old 10-29-2007, 05:14 AM
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Running a 225/70R-15 that measures 26.625" loaded height @ 35 psi.
I'm also trying to keep in mind the max speed available in 2nd gear when it downshifts going uphill towing. As the motor is stock w/ 90k miles, there's not much power on tap above 4600 even tho the engine is sound. Don't wanna tick a bunch of people off goin 40 mph in a 70mph zone.Thinkin the 3:43 may be the best compromise for my application.
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Old 10-29-2007, 06:02 AM
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With the OD trans, I would go with the 3.73
In OD you will have the equivalent of a 2.50 gear, about where you are now in third/drive.
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Old 10-29-2007, 06:29 PM
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I hear ya, 1ownerT, and if it weren't for the towing and not being able to tow in OD that's the way I'd go. I also need to make sure rpms don't get too high in second gear at, say, 60 mph or so when it downshifts from 3rd goin uphill. Here's some math for what I have now vs. other gearing:
2.56 rear in 2nd(1.63) = 3360 rpm @ 63.78 mph.
3.43 rear in 2nd = 4500 rpm @ 63.75 mph.
3.73 rear in 2nd = 4500 rpm @ 58.63 mph.
Just thinkin that since all the meat of the powerband is below 4500, I'd stand to at least slow the loss of speed climbing hills with the 3.43s. The chances of me pulling more than 4500 or so towing even with the 3.73s uphill is, I think, slim to none. And since hp above this rpm is actually on the decline rather rapidly, it would seem prudent to stay on the low side of this point.
If my thinkin is wrong please chime in and set me straight, I'm here to learn, and any and all feedback is welcome.
Thanks to all for helping me think this thing thru.
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Old 11-01-2007, 10:39 PM
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go with the 3.73's I just made that choice for my 85 caprice wagon. it has the 350 and a little bit warmed up. (and from the stock what i think were 2.76 gears. 2000rpm at 85.) It finally gets out of its own way now. and the burn outs are fun. and the wagon weighs, with a load at least as much as you car and the trailer and i drive it in od all the time, but if you do put the 350 in rebuild the trans with alto red eagle stuff. i love it in my car. ahem my wifes car.
good luck
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Old 11-02-2007, 02:38 AM
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Quote:
Originally Posted by runn141
it being an 86 caprice it should have an 8.5 rearend.

No, a regular sedan/coupe 86 Caprice came with a 7.5" 10bolt. The only way you got a 8.5" was in a wagon or a police package, trust me. And the rear in the wagon won't fit right on a sedan or coupe, its about two inches longer overall.
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