Ok, here it is....

I calculated my rear gear ratio using the the equation on the site here, and came up with a 3.42. I ran 60 MPH in third gear with the convertor locked up. I was turning about 2650, 2700 rpm. Tires are 26.1 in. tall. Now in the tech data section, it says that my car wasn't available with a 3.42. It's showing only a 3.23 available with a LG4 auto in 84.

Was a 3.42 available in 84, with a LG4 automatic, or is my speedo just off?

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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

 

If I remember correctly the equation asks for tire diameter not tire height. MAybe I am wrong

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I WOULD RATHER PUSH A CAMARO THAN DRIVE AN IMPORT

1983Z28 350w/ edelbrock performer RPM power package , dynomax shorty headers, 700R-4 with shift kit, 750cfm carb, edelbrock 3" exhaust system, ASCD SS hood, 16" IROC rims.

future mods:
completely done over suspension, black paint with flames, Hurst shifter, dakota digital gauge package, procharger supercharger.

 

Sounds like your tires are taller, causing your speedometer to read slower than it really is. I think stock tire size is 25.5" ? When your speedo says 60 MPH you're probably really going 65-67 MPH.

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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=-

 

If you are still having trouble, have a friend drive beside you and test there speedo to yours.

 

Tire height and diameter are the exact same measurement on my car. I think it just happens to work out that way on the tires that I got. It asks for radius anyway, so I had to measur from the center of the thing to the outside (actually I divided the height or diameter in half).

Anyway, the Z28 in 84 had 215/65-15 tires on it which are usually 26.0 inches tall. I went with the plus one size of 235/60-15 which has a height of 26.1. I didn't think that .1 inches would be a big deal since the tire will wear .5 or so over it's life (overall height).

THe main thing I want to know is if the 84 Z with the LG4 and auto trans came with 3.42's also, or just 3.23's.

------------------
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