New gears today

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May 17, 2011 | 09:42 PM

trainor

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Racer

Joined: Oct 2010

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From: Bishop CA

New gears today

Well today I had a set of 3.55 gears put in my 75, small block, turbo 400. As I was driving it in this morn i checked RPMs at 65 mph, they were at 2600 give or take. With the new gears the rpm has only gone up to maybe 2650 at same speed. It seems like there is a difference in bottom end but nothing that knocks my socks off. I think I should really notice a big difference. Any input?

May 17, 2011 | 10:42 PM

trainor

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Racer

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From: Bishop CA

Quote:

Originally Posted by trainor

C'mon gearheads give me some input, please

May 17, 2011 | 10:45 PM

SanDiegoPaul

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From: San Diego - Deep Within The State of CONFUSION!

your RPM change should be more than that. Were the old gears 3.08?

May 17, 2011 | 10:46 PM

72LS1Vette

Safety Car

Joined: Aug 2001

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From: North Easton Mass

What was the original gear ratio?

Rick B.

May 17, 2011 | 10:46 PM

DRIVESHAFT

Drifting

Joined: Aug 2005

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From: League City TX

Assuming you have 27" tall tires, something is wrong.
Assuming zero converter slip (or a manual trans) at 65 mph you should be turning close to 2900.

May 17, 2011 | 11:08 PM

mar

Burning Brakes

Joined: Dec 2007

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my 3.70's turn about 3000 rpm at that speed.

May 17, 2011 | 11:17 PM

TheSkunkWorks

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From: Graceland in a Not Correctly Restored Stingray

I second DRIVESHAFT's opinion.

May 17, 2011 | 11:38 PM

jjanow

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Posts: 51

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From: Illinois

Use a GPS and check your speed. You might find you are going slower that 65mph. The speedo gear in the trans may need changing for the new gears.

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May 17, 2011 | 11:41 PM

Vette5.5

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Joined: Sep 2004

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From: Livonia MI

Since this sound's like a fun post, I say 3300 rpm @ 70 mph with 3.55 axle and non lockup 1.00 auto top gear cruising. As many here, kinda been there before. What the heck "trainor", you asked for some advice, and came back wide ratio o.d. lock up tranny, but it's your car/money.

May 17, 2011 | 11:49 PM

#10

DRIVESHAFT

Drifting

Joined: Aug 2005

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From: League City TX

Quote:

Originally Posted by jjanow

The speedo gear in the trans may need changing for the new gears.

Thats your answer if you didn't change the gear in the transmission.

May 18, 2011 | 07:38 AM

#11

captanpants

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From: North Muskegon MI.

Spdo should be off 5-6 miles per hour

May 18, 2011 | 11:03 AM

#12

trainor

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From: Bishop CA

Quote:

Originally Posted by captanpants

Spdo should be off 5-6 miles per hour

I did have 3.08 gears in it before the change. I did check the mph before the change with a gps, so I will check it today after the gear change and see what I get.

May 18, 2011 | 12:35 PM

#13

scottyp99

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Joined: Mar 2008

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From: Oxford MA-----You just lost the game!!!!

Quote:

Originally Posted by trainor

I did have 3.08 gears in it before the change. I did check the mph before the change with a gps, so I will check it today after the gear change and see what I get.

Dude, the speedo cable comes off the back of the transmission, in effect it measures how fast the driveshaft is turning. It makes sense that if you change the diff gear, your speedo will be off. You could put a 5.13 back there, and the speedo would still say 65 at 2600 rpm........but you would actually only be going about 40 mph. Does it feel like you are going 65 mph? 3.08 to 3.55 might not be enough difference to notice by seat of the pants, but if you changed diff gear without changing speedo drive gear, then there is your problem. There are several ways to rectify this situation, including replacing the speedo gear in the tranny, a reducer that goes on the speedo line and effectively slows it down, new speedometer, changing back to 3.08 (Hah! just kidding, but it is an option.....) among other things. Hope this helps you get a handle on what's going on with your 'vette, see ya!

Scott

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