Axle ratio

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Pawned

Pawned

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I imagine that this has been asked and answered before, but I can not locate it.

To the best of my knowledge my 71 Duster 318V8-- A904 auto tranny and rear end are original to the car. There have been no modifications to any of them. The tires are 14" with a diameter of 24 inches.
Will someone tell me the Axle Ratio of this vehicle?
My speedo runs appx 13 mph high at 60 mph and I want to replace the speedo pinion gear to correct it, It came with a 29 tooth pinion
 
Put the rear axle on jack stands and turn the tire one revolution and count how many times the pinion yoke turned. That will give you a ballpark idea of the gear ratio and then use the chart to order the approriate speedo gear
 
You can go to mymopar and look at the manual for your car, in the transmission section there should be charts like the one below. I just changed my gear ratio which required speedometer pinion gear change as well. Using a GPS Speedometer ( i used speedbox, free app) agrees with my speedometer on the speed of the car. (So the chart below was accurate for me) If you have changed tire size - you can find several websites for that

upload_2016-9-10_6-53-9.png
 
racerdan80 said:
turn the tire one revolution and count how many times the pinion yoke turned
Click to expand...

For anyone who might read this, I'll point out that if you don't have a sure-grip differential, you must either turn both rear wheels together, or hold one wheel fixed while turning the other. With one wheel fixed, the pinion gear only makes half as many turns for each turn of the single wheel. For example, if one tire rev gives you 1.5 pinion yoke revs, then your ratio is 3.

Also, for more accuracy turn the tire several revolutions, count the yoke revolutions, and divide by the number of times you turned the tire. Example: 10 tire revs, 27 pinion revs => your ratio is close to 27/10 or 2.7 (now I'm assuming both wheels turned together).
 
The tires rotate in opposite directions. I turned the tire 10 times and the drive shaft turned 13.8 times

Am I doing something wrong. Or can it be the differential is not as stock as I think it is
I am kind of taking a guesstimate and could the axle ratio be 2.75

I read that one tooth will change the mph by 2-3 mph. I have ordered a 32 tooth to replace my 29 tooth pinion. Will let you know
 
Last edited:
2.76 was a common ratio in the stock 7 1/4 axles. That's what I had in my '74 Swinger.
 
rich006 said:
2.76 was a common ratio in the stock 7 1/4 axles. That's what I had in my '74 Swinger.
Click to expand...
Actually, 2.76 was my exact answer. I did not know it was a common ration until after I wrote that, so I just rounded it down. I am hoping the 32 tooth will be the perfect tooth count
 
This can be done mathematically because you posted all the required info,and it is the best kind of info.
These speedos are adjusted by percentages. since yours is running 13 miles high at 60, I'm gonna assume it reads 73 at 60, and that is 73/60= plus 21.67%
So your speedO pinion gear will need to slow down the needle this same 21.67%.That means it will need more teeth.
Since you currently have a 29 tooth, you will need; 29x 1.2167 =35.3 teeth

But if your speedo reads 60 at 47; then 60/47 =plus 27.7%. And you will need 1.277x29=37 teeth
 
AJ/FormS said:
This can be done mathematically because you posted all the required info,and it is the best kind of info.
These speedos are adjusted by percentages. since yours is running 13 miles high at 60, I'm gonna assume it reads 73 at 60, and that is 73/60= plus 21.67%
So your speedO pinion gear will need to slow down the needle this same 21.67%.That means it will need more teeth.
Since you currently have a 29 tooth, you will need; 29x 1.2167 =35.3 teeth

But if your speedo reads 60 at 47; then 60/47 =plus 27.7%. And you will need 1.277x29=37 teeth
Click to expand...
Thanks for the info.
I think 35 teeth would be way too high. Notice I said I think.
It seems the 32 tooth pinion will be here today or tomorrow. Which means I will get it installed probably by Thursday.
I will let you know how it turned out.
 
Pawned said:
Thanks for the info.
I think 35 teeth would be way too high. Notice I said I think.
It seems the 32 tooth pinion will be here today or tomorrow. Which means I will get it installed probably by Thursday.
I will let you know how it turned out.
Click to expand...
 
Ok I installed the 32 tooth pinion. The speedometer is reading 2-3 mph higher than my GPS said I am going.
If the speedo reads 65 mph, the GPS says 62 mph.
I imagine that a 33 tooth would have been perfect, but there would have been something else to throw it off.
So I am more than satisfied with the results I got.
I guess this thread can be considered solved, or not
 
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