I need help with speedo gears...which one do I need?
Thread Starter
Junior Member
Joined: Apr 2009
Posts: 11
Likes: 0
From: Medina, Ohio
Car: 89 Sport Coupe Vert (IROC-Z clone)
Engine: 383 Stroker TPI
Transmission: 700r4
Axle/Gears: 2.73 (3.73's awaiting install)
I need help with speedo gears...which one do I need?
Hello everyone, I have a 89 RS that currently has 2.73 rear end gears with a 700R4. I recently got a good deal on some GM 3.73 gears and now I need to find the right speedo gear combo. I looked at the GM part numbers on this site that are posted, but I`m embarrased to say that I don`t understand how to read them and I`m not very experienced in this field. I just want my speedometer to read the right speed after I have them switched out. I should say that the speedo gear is mechanical and not electric. Any help with this would be greatly appreciated.
Supreme Member
Joined: Oct 2004
Posts: 4,526
Likes: 238
From: Henrietta NY
Car: 1984 Trans Am L69
Engine: Sniper EFI Powered 355
Transmission: WC T5 w/ Steel Support Plate
Axle/Gears: 3.42 10 Bolt Posi
Re: I need help with speedo gears...which one do I need?
If only we had a sticky that told everything
https://www.thirdgen.org/forums/tran...ve-driven.html
Wait we do
https://www.thirdgen.org/forums/tran...ve-driven.html
Wait we do
"
The colors may not be exactly accurate, but I "believe" the tooth count is. Any additional info is appreciated.
3.23's up require a blue 7-tooth drive gear, plus the following driven gear:
18 tooth (p/n 3987918) Brown = 3.23
19 tooth (p/n 3987919) White = 3.42
20 tooth (p/n 3987920) Blue = 3.45
21 tooth (p/n 3987921) Red = 3.73
22 tooth (p/n 3987922) Gray =3.77"
The colors may not be exactly accurate, but I "believe" the tooth count is. Any additional info is appreciated.
3.23's up require a blue 7-tooth drive gear, plus the following driven gear:
18 tooth (p/n 3987918) Brown = 3.23
19 tooth (p/n 3987919) White = 3.42
20 tooth (p/n 3987920) Blue = 3.45
21 tooth (p/n 3987921) Red = 3.73
22 tooth (p/n 3987922) Gray =3.77"
Joined: Jul 2000
Posts: 6,519
Likes: 91
From: Aridzona
Car: `86 SS / `87 SS
Engine: L69 w/ TPI on top / 305 4bbl
Transmission: `95 T56 \ `88 200-4R
Re: I need help with speedo gears...which one do I need?
You didn't state your tire size.
If the tire diameter matches the tire diameter of one of these tire sizes, there you go.
If the tire diameter matches the tire diameter of one of these tire sizes, there you go.
Supreme Member
Joined: Oct 2004
Posts: 4,526
Likes: 238
From: Henrietta NY
Car: 1984 Trans Am L69
Engine: Sniper EFI Powered 355
Transmission: WC T5 w/ Steel Support Plate
Axle/Gears: 3.42 10 Bolt Posi
Re: I need help with speedo gears...which one do I need?
Here is an excel sheet I made to help someone determine what gear with different tires
Senior Member
Joined: Sep 2006
Posts: 573
Likes: 9
From: San Antonio, Tx
Car: 1988 Camaro IROC-Z
Engine: LB9 (305 TPI)
Transmission: 700R4
Axle/Gears: 2.73 Positraction
Re: I need help with speedo gears...which one do I need?
Speedo Driven Gear = [# of Transmission Tailshaft Gear Teeth x Number of Tire Revolutions Per Mile x Rear End Gear Ratio] / 1001
This will give you the correct number of teeth your speedo gear should have to be correctly calibrated. Sometimes an exact number cannot be reached, so make sure to round properly to account for the deviation so your speedo doesn't show slower than you are actually traveling.
This website has the calculator, which also show the amount of deviation if any on your speedo:
http://www.bgsoflex.com/speedo1.html
This website has a tire size calculator to determine your tire diameter in Inches:
http://www.miata.net/garage/tirecalc.html
Finally this site has both the first calculator, as well as the formula and much information about all the various 700R4 / 4L60 / T-5 / Etc speedo gears and setups. It has proven immeasurably helpful to me.
http://www.transmissioncenter.net/sp...n_______va.htm
This will give you the correct number of teeth your speedo gear should have to be correctly calibrated. Sometimes an exact number cannot be reached, so make sure to round properly to account for the deviation so your speedo doesn't show slower than you are actually traveling.
This website has the calculator, which also show the amount of deviation if any on your speedo:
http://www.bgsoflex.com/speedo1.html
This website has a tire size calculator to determine your tire diameter in Inches:
http://www.miata.net/garage/tirecalc.html
Finally this site has both the first calculator, as well as the formula and much information about all the various 700R4 / 4L60 / T-5 / Etc speedo gears and setups. It has proven immeasurably helpful to me.
http://www.transmissioncenter.net/sp...n_______va.htm
Joined: Aug 1999
Posts: 19,282
Likes: 103
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
Re: I need help with speedo gears...which one do I need?
JamesC
Supreme Member
Joined: Oct 2004
Posts: 4,526
Likes: 238
From: Henrietta NY
Car: 1984 Trans Am L69
Engine: Sniper EFI Powered 355
Transmission: WC T5 w/ Steel Support Plate
Axle/Gears: 3.42 10 Bolt Posi
Trending Topics
Joined: Aug 1999
Posts: 19,282
Likes: 103
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
Thread Starter
Junior Member
Joined: Apr 2009
Posts: 11
Likes: 0
From: Medina, Ohio
Car: 89 Sport Coupe Vert (IROC-Z clone)
Engine: 383 Stroker TPI
Transmission: 700r4
Axle/Gears: 2.73 (3.73's awaiting install)
Re: I need help with speedo gears...which one do I need?
Thanks fellas, that would have been painful without your help.
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