Muncie M 21 - ClassicOldsmobile.com


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Old February 21st, 2018, 09:39 AM   #1  
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Muncie M 21

I bought a M 21 1968 muncie 4 speed to install in my 67 cutlass and while putting in new seals and gaskets I pulled the speedometer drive assembly off and found no speedometer gear. After doing some research on the (inter-web) and found there maybe 2 sizes of drive gears 1.760 and 1.840 How do I know what one my car takes? or do I just get the right tooth count and they both fit? I maybe confused but these gears have a range of teeth 1.76 22 to 25 teeth the 1.84 a has 17-22 teeth So if i need a 18 tooth with 3:73 gears do I just get a 1.840 diameter gear. I understand the 1.760 is no longer made. I just need advice as to what direction to head and what fits my car.

Thank you in advance
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Old February 21st, 2018, 03:59 PM   #2  
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The drive gears are different diameters to compensate for the diameter of the driven gear. Both drive gears are 8 tooth so you need to determine driven gear teeth that you need and then select the approriate drive gear to go with it. You are correct in that the 18 tooth gear requires the 1.840 drive gear. I am not sure if you were just listing numbers for examples, but the 18 tooth gear is typically for lower axle ratios like 3.08. Depending on the tire size, 3.73 usually uses around a 22 tooth driven gear. There are actually two sizes of the 22 tooth gear available, one for each size of the drive gear. The 1.760 drive gears are available if you determine that is what you need. The TCI website has a great speeometer gear calculator. You can put in your tire size and it will calculate the diameter. Enter the tire diameter, your axle ratio and the number of teeth on the drive gear (8) and it will tell you how many driven gear teeth you need.

http://www.tciauto.com/tc/speedometer-gear-calculator/
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Old February 21st, 2018, 04:07 PM   #3  
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Thank you for the reply. I just threw those numbers out there, but without tearing the tranny apart how is an easy way to check what diameter gear I need 1.76 or 1.84
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Old February 21st, 2018, 04:31 PM   #4  
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The size of the drive gear 1.84 or 1.76 is based on the driven gear. First you need to determine how many driven gear teeth that you need. If you require a 17 to 22 tooth driven gear, then you need the 1.84 drive gear. If you require a 22 to 25 tooth driven gear, then you need the 1.76.
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Old February 21st, 2018, 05:41 PM   #5  
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Thank you for clearing that up for me, much appreciated. Now I know why everyone comes here because you get quality answers to common questions.

Thank You
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