Correct Speedo after Gear Change

Reply Subscribe

Thread Tools

Search this Thread

Aug 15, 2009 | 08:48 PM

CalvinMD

Thread Starter

Instructor

Joined: Apr 2009

Posts: 115

Likes: 0

From: Rising Sun Md.

Correct Speedo after Gear Change

What all is involved and must be changed to have the speedo read correct after a ratio change.....I am trashing the 2.59 ratio in favor of a 3.55 ring and pinion and want to make the speedo be resonably correct?...just a computer chip update? a gear in the tranny? or all of the above?/...or do you recommend another ratio for my 87 L98/automatic that will make correcting it easier to accomplish..

Last edited by CalvinMD; Aug 15, 2009 at 08:50 PM.

Aug 15, 2009 | 08:57 PM

aboatguy

Race Director

Joined: Jan 2005

Posts: 10,641

Likes: 13

From: Slidell Louisiana

Quote:

Originally Posted by CalvinMD

87 speedo calibration for gear change is by gear in tranny. http://www.transmissioncenter.org/GM...eter_Gears.htm
Due to the rpm limitations of the stock 5.7 TPI engine L98 guys usually go with something in the 3.08/3.07 neighborhood. Higher reving LT1 normal gear of choice is in the 3.45/3.55 neighborhood.

Good luck

Last edited by aboatguy; Aug 15, 2009 at 09:00 PM.

Aug 15, 2009 | 09:19 PM

rodj

Le Mans Master

Joined: Aug 2000

Posts: 8,838

Likes: 31

From: Australia

Quote:

Originally Posted by CalvinMD

the 2.59 ratio in favor of a 3.55

Change both the speedo drive gear and the driven gear in the trans WHILE the car is apart to change rear gears

See
http://forums.corvetteforum.com/1569009146-post2.html

Read
http://forums.corvetteforum.com/c4-t...or-3-73-a.html

Last edited by rodj; Aug 15, 2009 at 09:22 PM.

Aug 15, 2009 | 10:01 PM

CalvinMD

Thread Starter

Instructor

Joined: Apr 2009

Posts: 115

Likes: 0

From: Rising Sun Md.

Thanks guys...I'm pretty sure I want the 3.55's...the current engine is tired and will be 383ized with a better set of heads and bigger roller cam and chip to allow ez spinning up to 6200rpm

Aug 16, 2009 | 04:28 PM

aboatguy

Race Director

Joined: Jan 2005

Posts: 10,641

Likes: 13

From: Slidell Louisiana

Quote:

Originally Posted by CalvinMD

Thanks guys...I'm pretty sure I want the 3.55's...the current engine is tired and will be 383ized with a better set of heads and bigger roller cam and chip to allow ez spinning up to 6200rpm

What are you going to do for an intake (L98 intake is the rpm restriction at stock displacement)? Big heads and great valvetrain will not help with stock TPI intake.

Aug 17, 2009 | 12:30 PM

CalvinMD

Thread Starter

Instructor

Joined: Apr 2009

Posts: 115

Likes: 0

From: Rising Sun Md.

I have a whole new aftermarket replacement (from SLP) with larger ports and plenum and taller longer runners...along with a set of larger injectors and I'll be using a bigger BBK throttle body..this part I have figured out

Aug 17, 2009 | 01:06 PM

aboatguy

Race Director

Joined: Jan 2005

Posts: 10,641

Likes: 13

From: Slidell Louisiana

Quote:

Originally Posted by CalvinMD

Larger ports and plenum sound good but longer runners?

Good luck on your build!

Mike

Aug 17, 2009 | 01:27 PM

hbarclay

Heel & Toe

Joined: Jul 2008

Posts: 22

Likes: 0

From: Orange Texas

Quote:

Originally Posted by CalvinMD

When I acquired my 85 it had already been converted to 3.75 rear end. Otherwise stock L98 with automatic transmission. Although there was some vague reference to "head work" that was done on the car, it still has the original heads and intake.

Almost all of my driving is around town with just very occasional 15-20 minute highway driving. This is the only corvette I have driven so I have nothing to compare it to, but I love the acceleration I have available anytime I need it.

The speedometer had not been corrected and having to do the mental math all the time to know my speed finally got the better of me and I ordered this unit. Wired it in and calibrated it, and haven't had a problem since.

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