Just swapped an 85 mph cluster in my 85 IROC with a 145 mph cluster from an 87 IROC. Here is what I know so far. The tire sizes are the same (245/50/16), the trans' are the same (700R4), my rear gear is 3.42 standard, the gear in the 87 is 3.23 posi.

In my tailshaft I have a 15 tooth drive gear with a 41 tooth driven gear (2.73 ratio). I have been told that I may have to go as low as a 36 tooth driven gear, but don't know if it is large enough to contact the current drive gear.

Does anyone know the formulas to calculate this or have an 87 IROC with 245/50/16 tires, 700R4, 3.42 gears, 145 mph cluster, that may know what trans gears they have.



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Rob Kowalski
85 IROC, 305 TPI, JET AFPR - 45 PSI, B&M Megashifter, B&M Shift Kit, 3.42 gears, SLP Airfoil, SLP 1 5/8 headers and cat back system, gutted cat, gutted airbox, MAF screens removed, K&N, more to come!!

 

The cluster are all calibrated for 60 MPH with a 1000 rpm input. You don't need to change a thing! I didn't!

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Working on:
'84 Z28 LG4 305 with 200,000 original miles!
Added dual elec fans.
145 MPH IROC Speedo
Building 430 HP 350 (ZZ430)
using primarily GMPP parts.
Short block sitting on a stand. (Man, those Fast-Burn heads sitting on it look good!)

Starting to look like the Kicker poster child!

ASE Certified Master Tech

 

You definitely dont need to change a thing.

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1989 IROC-Z 5.7L
NOS 150HP kit
ProBuilt 700R4, PI Vigilante 2800 stall lockup
Baer Brakes 12" Sport System

 

drive driven
15 41
16 44

While your speedo is calibrated the same as your old one...it needs these gears in the tailhousing to be accurate...these are available thru the dealer...but looking at your setup you already have the correct gears...so you are all set to go

[This message has been edited by AfootVoyager (edited May 12, 2001).]

 

You only need to change the speedo gears in the trans if you change the gears in the rear end. If you're just swapping the speedometer, you dont have to change anything else.

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1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.23 @ 107.62 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
Webmaster: SoCal F-Bodies
-=ICON Motorsports=-

 

Or if you change the height of your tires.

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1989 IROC-Z 5.7L
NOS 150HP kit
ProBuilt 700R4, PI Vigilante 2800 stall lockup
Baer Brakes 12" Sport System