# Speedo Gear Selection help



## EdR (Sep 2, 2014)

Hi.. I'm changing my rear end gearing in my 69 GTO.(4 speed Muncie) Not even sure what rear end gearing I have now, but its stupid low.. like 4th gear 3500 rpm and 40 MPH. Anyway.. I want to change the rear end gearing to 3.42 with 255/60R15 tires (27" diameter). Any help with what I need to do with speedo gearing so its accurate? Is there some other piece to puzzle I need like gearing that's in the tranny that I need to know 1st?


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## EdR (Sep 2, 2014)

I think I figured out that its and M21 Tranny. (10 spline and one grove on the input shaft) Ratios 1st thru 4th: 2.20 / 1.64 / 1.28 / 1.00 although it looks like they are all 1.00 in fourth so idk if that really matters.


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## azmusclecar (Sep 26, 2007)

There are plenty of websites with tools to use to figure it out. I like several and use different ones to play with different set ups.

It's as simple as plugging in your numbers. If you get stuck, PM me and I will help you out. I'm in the process of trying to match a 3.08 rear gear for highway use with the proper transmission and there are some that do NOT go together.

Here's some websites............

Transmission Gear Ratios

Calculators

Muncie 4-speed Transmissions | Hemmings Motor News

Muncie Transmission Id and ratio guide

Speedometer Gear Chart

4Lo.com :: Tire Size Change, New Gear Ratio Calculator

http://www.dandltransmission.com/pdf/t_10.pdf


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