C3 Tech/Performance V8 Technical Info, Internal Engine, External Engine, Basic Tech and Maintenance for the C3 Corvette

Speedo Question

Old 03-11-2019, 09:10 AM
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Default Speedo Question

Hi all, long time lurker here. My dad's got a '68 Roadster with 427/4-speed that recently lunched the stock rear end. We got pop a new rear end for his birthday and everything went in and we're back up and running as of 6PM last night. His stock(or at least what was in the diff) gears were 4.11:1, and we went back with 3.55:1 to pair up with the 4-speed to make it a little more highway friendly. My question is, how do we calibrate the speedo to the new gear ratio? Do we even need to do this? Thanks in advance!!!!
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Old 03-11-2019, 09:20 PM
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I forgot to post that I found the right chart... lol.. god I'm old.

I posted you a chart below for AT cars in error, for some reason the one I have for the MT car's isn't showing up on my tech site.... Then I found it... lol..

The calibration on all speedos is the same, it's the driven gear in the tranny that makes the difference. Use the chart below and you'll know which driven gear you need to switch to correct the calibration issue.


Last edited by Willcox Corvette; 03-14-2019 at 03:49 PM. Reason: third edit... found the right chart.
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Old 03-14-2019, 09:02 AM
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There are two types of driven gear, the diameter where the teeth is deferent.
In my Muncie transs' 4:11 the driven gear is large type.
Small type: 20 millimeter ~ 0.8 inch (Diameter).
Large type: 22 millimeter ~ 0.87 inch (Diameter).
I didn't found in the market the large type so I put adapter when I changed my rearend.

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