Speedo gear 1970 A904 selection

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lomchivok

lomchivok

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I have 2 chunks I'm gonna use in my 8 3/4...1 is a 3.23 and the other is a 3.55. I need to figure out which speedo gears are correct for my 1970 A904 as I am finding conflicting info.
I think these are the ones... for the 3.23 P/N 2838931(Green)
and for the 3.55 P/N 2538933(Yellow)...info from mymopar.
Can y'all help me to determine if this is correct for a 255/60 15 bfg tire, 17.1" dia.
 
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looks like its a 31 and a 34 tooth respectively.
 
lomchivok said:
looks like its a 31 and a 34 tooth respectively.
Click to expand...
I did the math for your tire diameter, and it came out at about 27.0 inches (it could really be rounded to 27.1 if that makes a difference). I used a chart on drivinithome.com, and it only shows diameters of 26.8 and 27.4. The value 26.8 technically is closer to 27.0 than 27.4... if you were to go by that you'd want a 32 tooth and a 35 tooth.

IMG_0137.JPG
 
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Mike Correll said:
I did the math for your tire diameter, and it came out at about 27.0 inches. I used a chart on drivinithome.com, and it only shows diameters of 26.8 and 27.4. The value 26.8 technically is closer to 27.0 than 27.4... if you were to go by that you'd want a 32 tooth and a 35 tooth.

View attachment 1715070280
Click to expand...
But I think either way it'd be pretty close... I went by drivinithome.com's chart for my 66 Barracuda with 3.23 gears. With my math, my tire diameter came out to 25.6. That diameter and ratio called for a 33 tooth speedometer gear, so that's what I put in. Turned out really accurate.
 
Thanks for the response. The info I found for my tire said 27.1" and I wrote 17.1"... My bad. Any idea what the diff is with the short and long shaft gears...I assume my 70 904 has the long shaft???
 
metric tires are rounded up to the next nearest size. A 255 could be as small as 253 or as large as 257, when mounted on a checking-rim that is 70% of the branded size,rounded to the nearest 1/2 inch. So 70% of 255 would be 7 inch rim.So in reality, a 255/60-15 could be as large as 27.14, and as small as 26.95. However, this is less than .7% difference, or .45mph at 65.
The gears can usually get you to within 2% of perfect, which is about 1.3 mph at 65 mph. So at a true 65 the gauge could be indicating somewhere between 63.7 and 66.3.And the tire diameter could kick in it's own error.

One trick I have used, is to just throw something close in there and go for a ride.Then as opportunity presents, I calibrate the current error in percent. This could be done on one of those calibration sections on the hiway, or by radar, or by gps,etc. The % error can be directly applied to the current gear.So if you had a 3% error on a 33 tooth gear this would be a 32.01 tooth or a 33.99 tooth requirement, depending on which way the error was. From that one could surmise that a 1.5 % or less error is not addressable.More teeth slows the speed-O down
 
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