Transmissions and Drivetrain Need help with your trans? Problems with your axle?

speedo after 4th gen rear end swap

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Old Jan 27, 2007 | 07:00 PM
  #1  
ajk511's Avatar
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From: Libertyville, IL
Car: 1991 z28
Engine: L98
Transmission: 700r4
speedo after 4th gen rear end swap

im about to swap a rear end from a 99 z28 into my 91 z28, since the gears are different is my speedo going to be off after the swap?
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Old Jan 27, 2007 | 07:19 PM
  #2  
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From: Detroit, MI
Car: 1991 Z28 Vert
Engine: LT1 350
Transmission: T56 6 Speed
Axle/Gears: 3.42 LSD
Yes your speedo will be off.

Jay
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Old Jan 27, 2007 | 10:15 PM
  #3  
ajk511's Avatar
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From: Libertyville, IL
Car: 1991 z28
Engine: L98
Transmission: 700r4
ok. anyone know what i have to do to get it right again?
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Old Jan 27, 2007 | 10:22 PM
  #4  
ajk511's Avatar
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From: Libertyville, IL
Car: 1991 z28
Engine: L98
Transmission: 700r4
sorrym just did a search and founs some answers. anyone know what gears were in a 99 z28 and what size speedo gears i will need to replace mine with?
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Old Jan 28, 2007 | 06:43 AM
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From: Lawrence, KS
Car: Met. Silver 85 IROC/Sold
Engine: 350 HO Deluxe (350ci/330hp)
Transmission: T-5 (Non-WC)
Axle/Gears: Limited Slip 3.23's
Originally Posted by ajk511
anyone know what gears were in a 99 z28 and what size speedo gears i will need to replace mine with?
Here's how to determinne what ratio the rear is (jerk the diff cover and look): numbers are stamped on the ring and pinion:

41 : 15 = 2.73
40 : 13 = 3.08
42 : 13 = 3.23
41 : 12 = 3.42
41 : 11 = 3.73

You may have to swap the drive/driven gears in the transmission--those will depend on the what tranny you're running, auto or manuel. A search should provide more info.

JamesC

Last edited by JamesC; Jan 28, 2007 at 06:46 AM.
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Old Jan 28, 2007 | 10:29 AM
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From: Orange, SoCal
Car: 1990 Pontiac Trans Am
Engine: 355 TPI siamesed runners
Transmission: Tremec T56
Axle/Gears: 12-Bolt 3.73
Originally Posted by ajk511
sorry just did a search and found some answers. anyone know what gears were in a 99 z28 and what size speedo gears i will need to replace mine with?
A 99 Z28 could come with 2.73 gears (auto), 3.23 gears (auto), or 3.42 gears (stick). A search of this forum will tell you what speedo gears you need in your transmission.
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Old Jan 28, 2007 | 10:34 AM
  #7  
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From: Libertyville, IL
Car: 1991 z28
Engine: L98
Transmission: 700r4
thanx guys
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Old Jan 28, 2007 | 12:25 PM
  #8  
ajk511's Avatar
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From: Libertyville, IL
Car: 1991 z28
Engine: L98
Transmission: 700r4
is there an easier way than changing the speedo gears in the trans everytime i switch gear sizes or tire sizes? can i get an electrical vss to hook up to my speedo or somthing that will let me program in gear and tire size and respond accordingly?
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