Gear Ratio Change/Speedometer Correction

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Aug 9, 2006 | 11:04 AM

vette punk

Thread Starter

Racer

Joined: Apr 2001

Posts: 322

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From: Sherburne,NY NewYork

Gear Ratio Change/Speedometer Correction

I am planning on a rear axle gear ratio change from 3.33 to 3.73 on my 89 C4 6 speed coupe and
need information and parts sources to correct the speedometer. How
is this done? Who sells the parts to correct the speedometer?
Thanks,
Jim B.

Aug 9, 2006 | 12:11 PM

toptechx6

Safety Car

Joined: Jun 2006

Posts: 3,629

Likes: 28

From: FL

Two options

Quote:

Originally Posted by vette punk

From what I've been told there are two ways you can go, see links below. I recently bought 3.73's and will likely change the trans gears. If the links won't work directly copy and paste into your browser search field.

http://www.ikerds.com/merchant2/merc...t_Code=400-083

http://www.dakotadigital.com/index.c...rod/prd126.htm

Aug 9, 2006 | 12:53 PM

tequilaboy

Melting Slicks

Joined: Aug 2003

Posts: 3,399

Likes: 395

From: Lakeville MI

The combination of 15 and 45 teeth gears ought to do the trick for 3.73s and normal tires.

Aug 9, 2006 | 08:41 PM

CFI-EFI

Race Director

Joined: Sep 2000

Posts: 17,298

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From: The Top of Utah

Quote:

Originally Posted by toptechx6

From what I've been told there are two ways you can go, see links below. I recently bought 3.73's and will likely change the trans gears.

Changing speedo gears is MUCH more economical, and less work in your case. HERE is a tooth count calculator. Presently, your drive gear has 15 teeth. There should be no reason to change it. The calculator says to use a 43 tooth driven hear with 26" tall rear tires. You can buy these gears a lot cheaper at your Chevy dealer. The 43 tooth driven gear is part number 25513050. The 44 tooth gear is 25513051, and the 45 tooth gear is 25513052. Unlike some other ratio swaps, you will not have to change the drive gear or the VSS.

RACE ON!!!

Aug 10, 2006 | 06:16 AM

toptechx6

Safety Car

Joined: Jun 2006

Posts: 3,629

Likes: 28

From: FL

Quote:

Originally Posted by CFI-EFI

Thank you! You just saved me some money and aggravation, I was somewhat confused about the VSS, thanks for the clarification. Under what circumstances is changing the VSS required?

Aug 10, 2006 | 11:55 AM

CFI-EFI

Race Director

Joined: Sep 2000

Posts: 17,298

Likes: 33

From: The Top of Utah

When the driven gear crosses the 39 tooth mark. It is similar to using a 2 series diff case with 2.59, 2.78, etc. gears, and a 3 series case with 3.07, 3.33, etc. gears. One VSS fits 35 to 39 tooth driven gears and the other fits 40 through 45 tooth driven gears. With vette punk changing from 3.33 gears to 3.73s, he had a 40 tooth driven gear. He won't be crossing that 39 tooth point when he swaps in the 43, 44, or 45 tooth driven gear.

RACE ON!!!

Aug 10, 2006 | 12:19 PM

toptechx6

Safety Car

Joined: Jun 2006

Posts: 3,629

Likes: 28

From: FL

Quote:

Originally Posted by CFI-EFI

Got in now, thanks for the explanation.

Aug 10, 2006 | 12:42 PM

sami85L98

Le Mans Master

Joined: Apr 2005

Posts: 8,077

Likes: 4

From: Scarborough ONTARIO

CFI-EFI helped me correct my speedometer correction, before i ask CFI i already bought n installed Dakota digi box that helped me correct speedometer but when i installed new tranny, at the same time i installed CFI's recomended combo and very happy with the result, now no need of dakota box. I bought these 15 and 43 from transmissionhead.com.