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What size Posi...

6

6T3Vette

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How can I tell what my Positraction gear ratio is w/o actually counting the teeth? I have a non-original '63 convertible, 4 spd, 300hp, 215x70 tires. I have read that stock was mostly a 3:36 for that engine but I'm pulling about 3200 rpm at 60mph in 4th gear. Does that sound right for a 3:36? While I enjoy the get-up-and-go, I would prefer more economy on the highway. If indeed I do have a 3:36, would a 3:08 make much of a difference on the rpm's?

I am also noticing a distinct "thud" coming from possibly the rear end when I upshift. Could this possibly be u joints on the drive shaft, half shafts or both?

Thanks for your help
 

Tom Bryant

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Location
Edgerton, Ohio, United States
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1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
3.36 rear

Hi Terry,

I have had both a '65 and a '67 with 3.36 gears. At 70 mph you will be turning 3000 rpms. My '65 300hp would get 18.5 to 19 mpg on a road trip back when the speed limits in IN were 70. you must have 3.70 or even 4.11 gears. I don't think that you would like 3.08s with the 327. You will have to slip the clutch to get it moving and it will be dead off the line.

I would check the u-joints first. That is the most likely cause of your thud. My '65 had excessive backlash in the ring and pinion gears and would thud when I applied the gas and when I backed off the gas. Very annoying.

Tom
 

Tom Bryant

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7,304
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
I just ran your figures through the rear end ratio calculator in the Knowledgebase and it appears that you have a 4.11 gear.

Tom
 
6

6T3Vette

Guest
Tom,

Thanks for the info. I may consider putting a 3:36 in "Miss Georgia" for better economy. Looks like for now it's time to inspect those u-joints. I hope it's the driveshaft and not the half shaft.

Cudos to you and the team at CAC. I'm a relatively new vette owner and find your site very informative. I enjoy learning about and working on her just as much as I do driving her. Your site is a great resource. Thanks again.
 

Tom Bryant

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7,304
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Edgerton, Ohio, United States
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1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
Thanks for the complements. We would enjoy seeing a picture of your '63 sometime.

Tom
 

WayneC

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Jan 5, 2002
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472
Location
Santa Barbara CA
Corvette
63 roadster project
Also check the cushions on the nose bracket of the differential .... if the bushings are tired, it will allow the differential bracket to have too much play and/or to come up too high, allowing the U-joints to strike the frame bracket or the body on hard acceleration, Aftermarket poly bushings are often too thin, so best to replace with the correct rubber GM bushings
 
6

6T3Vette

Guest
Thanks Wayne. I wasnt aware that some poly bushings may be thinner than original GM's. I'll be sure to check it all while I'm under her.

Tom....attached is the pic of Miss Georgia you requested. I bought her from a gentleman in Georgia last spring, hence the name. He had her painted in '87 and kept her in the garage since. Like I mentioned before, not all original (engine and some parts are '64) but she's a great driver and definitely an eye grabber.
 

Tom Bryant

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Edgerton, Ohio, United States
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1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
That is a sweet ride. It's the same color combo as my '65 was.
 

Ol Blue

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238
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usa
what size posi - and clunk

6T3
If your 63 has the original nose bracket that connects the front of the differential to the crossmember, it could be the holes in side of the bracket are slotted from fast starts. Later cars had reinforcements in that area.

There is one bolt that goes all the way through the Diff and bracket, and then a bolt on each side that bolts to the Diff. Most of the hard drivers that I ran around with in that era slotted those holes pretty quickly. It started out as a clunk, then if not taken care of the U-Joint will begin hitting the tunnel and eat into that.

Ol Blue
 

Glassman

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Sep 9, 2001
Messages
42
Location
Mentor, Ohio, Lake
Corvette
84, primer /69 brite red /66 red / 60 hugger orang
My bet is that your pinion snubber bushing is shot it causes the steel bracket to jump up and down when on and off the gas.

To fined your gear ratio jack up the car and place a chalk mark on the differential and pinion then mark the tire and line it up to a stationary point then turn the tire making sure the other tire is moving at the same time then count the revolutions of the drive shaft you can easily figure the ratio. for 3:70 the drive shaft will turn 3.7 times.
 

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