# Which speedometer gear?



## Bob 69 vette (Sep 18, 2021)

1967 GTO, 400, 4 spd. I pulled the diff cover and determined that I have a 3.36 ring and pinion. Question: with the Muncie 4 spd and 3.36 gears, how many teeth (or what color) speedo cable gear (the gear that drives the speedo cable and is mounted in the hole in the side of the trans) should I be using to get an accurate MPH reading? Thx, Bob


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## Sick467 (Oct 29, 2019)

You will need to know what tire diameter you are running in the back. This can be tricky since tire and wheel combinations can change the actual tire diameter that online calculators would have you think. I have found that if I measure from the center of the wheel down to the floor (all the weight on the car) and multiply it by 2...I get a good number to work with. If your measurement is hard to get exactly to an 1/8th inch...I bump it to the slightly higher measurement since tires do swell up at higher speeds, ever so slightly. So, knowing your tire and rim size will allow for an online calculator to produce an diameter, but none that I have found account for tire squat from the weight of the car. It's best to measure it. Online numbers can get you there, or close, but close may mean you have to run with a speedo that's off a bit or buy a different gear.

You will also have to know the speedo drive gear tooth count. I am not familiar with the Muncie 4 speed...did it come with optional drive speedo gear sizes?


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## 1971LemansWisc (Apr 18, 2021)

Bob - 

I have Muncie Wide Ratio M20, 3.36 Gears, Tires P225/R7015

Site TCI Speedometer Gear Calculator

Under - Speedometer Gear Calculator

# of Drive Teeth: 8

Axle Ratio: 3.36

Tire Diameter: 27.4 

# of Driven Teeth: 19.7654

Closest Driven Gear is 20 Teeth - Blue


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