speedo pinion gear selection

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HemiDenny

HemiDenny

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speedo is off about 5mph........... @35 it reads 40, @ 55mph it reads 61.

currently I have a 38 tooth pinion which is what Brewers chart shows for a 27" tall tire (275 / 60 / 15) with a 3:91. I verified it has 3.91's and installed the (blue) 38 tooth speedo pinion myself.

I'm thinking a 39 or 40 tooth speedo pinion. Can anyone take the guess work out of it for me?

Thanks,
Denny
 
junkyard trick was the teeth X.1 to get the right rear end ratio assuming the tires were stockish. try a 39. More teeth is a slower rotation, ie slower reading at same road speed.
 
If you have the math dialed in, is your speedometer pinion gear housing clocked correctly?
 
Mph on speedometer
Mph on GPS / phone.

Devide the two gives you a ratio

Multiply your current gear teeth count by the ratio and that will give you the new teeth count.

You will have to figure out if you devide the speedo by the gps or the other way around. ( I just don't remember and am brain dead right now)

This method does not need rear ratio or tire size.
 
Last edited:
Alaskan_TA said:
If you have the math dialed in, is your speedometer pinion gear housing clocked correctly?
Click to expand...

pretty sure. but will re-check. however, I'm not sure that would make any difference other than tearing the plastic gear up. But I have been wrong before, just ask my wife.
 
HemiDenny said:
speedo is off about 5mph........... @35 it reads 40, @ 55mph it reads 61.
Click to expand...
You have two problems;
from 34 to 40 is a 14.38% difference, but
from 55 to 61 is only 10.90%

When a speed-O is out, it is always a linear progression.
When changing rear gears, the new Speed-O tooth count is always the same % difference as the gear% change.
so like going from 3.55s to 3.91s is 3.91/3.55= plus 10.14%, which would require a tooth difference of the same 10.14% .. Since the 3.91s are now gonna spin the driveshaft faster for a given roadspeed, the speed-O will be faster, so you gotta install more teeth to slow it down. So if your 3.55s had a speed-O with xx number of teeth, the 3.91s would require; xx times 1.1014..
The numbers will typically only get you to within about 2.6%.
For instance if your combo requires a 1.5% change at 39 teeth; then 40 is 40/39=1.0256; which is a difference of ~2.6% whereas
38/39 is .9744; a difference of 2.6% again. In either case. your Speed-O will remain out be 2.6 less 1.5= 1.1% ...........
In your case; you are getting two conflicting change requirements, namely 14.38% and 10.9%. So IDK: maybe your cable needs lubricating first; IDK.
 
AJ/FormS said:
You have two problems;
from 34 to 40 is a 14.38% difference, but
from 55 to 61 is only 10.90%

When a speed-O is out, it is always a linear progression.
When changing rear gears, the new Speed-O tooth count is always the same % difference as the gear% change.
so like going from 3.55s to 3.91s is 3.91/3.55= plus 10.14%, which would require a tooth difference of the same 10.14% .. Since the 3.91s are now gonna spin the driveshaft faster for a given roadspeed, the speed-O will be faster, so you gotta install more teeth to slow it down. So if your 3.55s had a speed-O with xx number of teeth, the 3.91s would require; xx times 1.1014..
The numbers will typically only get you to within about 2.6%.
For instance if your combo requires a 1.5% change at 39 teeth; then 40 is 40/39=1.0256; which is a difference of ~2.6% whereas
38/39 is .9744; a difference of 2.6% again. In either case. your Speed-O will remain out be 2.6 less 1.5= 1.1% ...........
In your case; you are getting two conflicting change requirements, namely 14.38% and 10.9%. So IDK: maybe your cable needs lubricating first; IDK.
Click to expand...

cable is new and was lubed when installed.

I have come to the conclusion after every ones valuable input, reading another thread and several speedometer charts / gear calculators....the 38 tooth is correct but the speedometer is likely in need of a rebuild and is off. I will check and see if the odometer is off also just for giggles.

Thanks for all your input
 
Last edited:
IMHO pick the speed you want the speedo to be the most accurate at and shoot for that.

Also would you rather it read a little too high or a little to low.
 
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