What gear ratio

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Dec 3, 2005 | 10:06 PM

1978 Pace Car

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What gear ratio

Tonight I got my car up to 80mph for the first time! When I did my tach was reading 5500 rpm. It was in the yellow line of my tach, so what gear ratio do I have guys?

Thanks guys: Gene

Dec 3, 2005 | 10:13 PM

76mike

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From: Sunnyvale CA

I think that I have 4.11 gears and mine isn't that bad... you're either running higher than that or you have small tires.

Dec 3, 2005 | 11:24 PM

supercalvin

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From: Boise Idaho

WOW, What gear was the tranny in?

Dec 4, 2005 | 01:13 AM

Older Than Dirt

Burning Brakes

Joined: Aug 2002

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From: Scottsboro Alabama

If your tire diameter in height is.......
25" your rear end ratio is 5.12
26"............................. 5.32
27"............................. 5.52

Dec 4, 2005 | 01:54 AM

fauxrs2

Drifting

Joined: Feb 2005

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From: San diego ca

Quote:

Originally Posted by 1978 Pace Car

Tonight I got my car up to 80mph for the first time! When I did my tach was reading 5500 rpm. It was in the yellow line of my tach, so what gear ratio do I have guys?

Thanks guys: Gene

Well the Steepest gear available in 78 was a 3.70:1 gearset....

If you have the factory gearset then you must have insamely tiny tires...or your Tach is wrong...

Dec 4, 2005 | 02:04 AM

63C2splitter

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From: Central Plains

St. Jude Donor '08

Something isn't right. Is it an auto or standard transmission? If an automatic, is it getting into high gear?

My 63 has a 4:11, and at 80 it tachs at about 4200. my tire height is ~27 inches.

Dec 4, 2005 | 07:31 AM

Glensgages

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From: State of Confusion

Quote:

Originally Posted by 63C2splitter

My 63 has a 4:11, and at 80 it tachs at about 4200. my tire height is ~27 inches.

When I ran a THM400 and 26" tires on my Z28, 85 MPH was approx 4800 RPM.

To accurately compute your gear ratio, we'd need to know what size (height ) your rear tires are, and what type of transmission (THM350/400, A/OD, or standard shift ) you have, and if your tach is functioning properly:
after-that, it isn't too-difficult.

Dec 4, 2005 | 11:43 PM

1978 Pace Car

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Im having a hard time beliving it myself! I will measure my tires in the mourning! I counted the gears when my car shifts and it goes all the way up to third gear with no problem, I just dont understand why it seems so low! It doesnt seem right! Thats for sure.

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Dec 4, 2005 | 11:46 PM

Stoge

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From: Fair Oaks, OK

I don't know anything when it comes to this stuff, but is it possible that his torque converter is messed up?

Dec 5, 2005 | 01:20 AM

#10

Glensgages

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From: State of Confusion

Quote:

Originally Posted by Stoge

I don't know anything when it comes to this stuff, but is it possible that his torque converter is messed up?

..... this is another possibility.

Dec 5, 2005 | 04:56 AM

#11

camaroneil

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From: texas,, at last

could it not be his speedo,
he was maybe doing 120 mph !!!!!!!!!!!!!!!!!!!!

i know my speedo is real sluggish, especially when cold,

natt

Dec 5, 2005 | 07:18 PM

#12

1978 Pace Car

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Ok, heres what I have for tires on the car, their P225/60 R15. And they measure at 24 3/8" tall. Whats that sound like to you guys now? Is it the tires or the the speedo, or torq converter?

Thanks guys: Gene.

Dec 5, 2005 | 09:47 PM

#13

Glensgages

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From: State of Confusion

Quote:

Originally Posted by 1978 Pace Car

Ok, heres what I have for tires on the car, their P225/60 R15. And they measure at 24 3/8" tall. Whats that sound like to you guys now?
Is it the tires or the the speedo, or torq converter?

..... you didn't even consider that the tach could-be off.....

Using a formula that I've used for years to compute RPM/MPH/tire-height/gear-ratio, here is what we know:
RPM: 5500
MPH:80
tire-height: 24.375

"assuming" a 10% slippage in the torque-converter (typical of THM350/THM400 ), this is what the calculation would be:
RPM (5500 ) x.9 (10% slippage ) x .006 (a 'factor', that includes pi ) x 1/2 tire-height (12.187, in this instance ) divided-by MPH (80, in this instance ) = GR (gear-ratio ).

5500 x .9 x .006 div/by 80 = 4.52, or close-enough to be considered 4.56 gears:
I'd still jack-up the car, spin the tires 10 revolutions while counting drive-shaft revolutions, and divide drive-shaft revolutions by 10.

Last edited by Glensgages; Dec 5, 2005 at 09:49 PM. Reason: own-stupidity

Dec 5, 2005 | 10:01 PM

#14

1978 Pace Car

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Thank you very much!!!!!!!!!!!! That would suck if I did have gears like that in this car! But I will jack up the car this week if it doenst snow on us and do what you told me to do.

Thanks again: Gene.

Dec 5, 2005 | 10:13 PM

#15

Glensgages

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From: State of Confusion

Quote:

Originally Posted by 1978 Pace Car

That would suck if I did have gears like that in this car!

Maybe, maybe-not..... if you DO change the gears, and you DO have .56s, I understand that they are very hard to find any-more, and may-be worth a few scheckels.....

With 4.56s, that thing oughta SCREAM off the line, if it hooks at-all, even-if it is a wimpy L-48!

Quote:

Originally Posted by 1978 Pace Car

..... I will jack up the car this week if it doesn't snow on us and do what you told me to do.

Put a chalk-line on the very-bottom of the drive-shaft, and a chalk-line on the inside of the wheel/tire, from the bead to the tread, at the very-bootom, as-well:
spin the tire exactly (or very-close to exactly ) 10 revolutions while counting drive-shaft revolutions (full AND partial revolutions ).

33+ revolutions would be 3.31s or 3.36s, 35+ would be 3.55s, 37+ would be 3.70/3.73, 41+ would be 4.10/4.11, 45+ would be 4.56s, etc.

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