Speedo gear and rear gear

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Slantsix64

Slantsix64

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i have a 3.23 ratio speedo gear but my rear end gear is a 2.93 how much would my mileage and speed be off?
 
Assuming everything else is the same....Approx 10% slow if my calculations are correct.
How i came to this conclusion...
If you turned 1500 rpm at 60mph with 3.23 rear gear and correct spedo gear. With a 2.93 rear ratio at the same 60mph you would be turning 1350rpm so the speedometer would show about 54mph
 
There is no such thing as a 3.23 speedometer gear though.

One size (gear tooth count) will not be accurate with all 3.23 rear gear applications.

So the rear tire diameter has to also be known.
 
With no other changes 2.94/3.23=91%= 9 percent..
From a 24.5" tire to a 27" you can have another error of up to 10% (27/24.5= 1.102)
Technically speaking it could be out anywhere from zero to 19%, depending on the actual tooth count.
The best and fastest way to figure out the error of the speedO versus real roadspeed, is to figure it in percent, and to change the pinion by the same percent.
To find real roadspeed; you can use GPS, or mileage markers on a hiway set up for that, or you can use mile-markers and a stopwatch. One mile should take 60 seconds, keep changing your speed until it does, then calculate the difference from what the speedO is displaying to 60mph.
Or record the actual seconds and compare it to 60.
Example 1;
the speedO displays 54 mph at a true 60 mph. Your speedo is displaying 54/60=.90= 90%. You need to increase it 10%.
Example 2;
You drive 5 miles in 270seconds which is 54 seconds per mile. 54/60=.9 or 90%, and you need to increase it 10%
So
you pull out the pinion gear and count the teeth, and find 33teeth. to increase the reading you need to spin the cable faster so you need fewer teeth, subtracting 10% from 33 you get 30.
Your numbers may be different but the percent rule is never wrong.

Or you can do what probably most of us do, which is drive thru a radar zone and determine our % error and after that,just do the mental computations for the various speed zones we encounter on a day to day basis. Ten percent is easy math lol.
 
Last edited:
AJ/FormS said:
With no other changes 2.94/3.23=91%= 9 percent
from a 24.5" tire to a 27" you can have another error of up to 10% (27/24.5= 1.102)
Technically speaking it could be out anywhere from zero to 19%, depending on the
actual tooth count.
The best and fastest way to figure out the error of the speedO versus real roadspeed, is to figure it in percent, and to change the pinion by the same percent.
To find real roadspeed; you can use GPS, or mileage markers on a hiway set up for that, or you can use mile-markers and a stopwatch. One mile should take 60 seconds, keep changing your speed until it does, then calculate the difference from what the speedO is displaying to 60mph.
Or record the actual seconds and compare it to 60.
Example 1;
the speedO displays 54 mph at a true 60 mph. Your speedo is displaying 54/60=.90= 90%. You need to increase it 10%.
Example 2;
You drive 5 miles in 270seconds which is 54 seconds per mile. 54/60=.9 or 90%, and you need to increase it 10%
So
you pull out the pinion gear and count the teeth, and find 33teeth. to increase the reading you need to spin the cable faster so you need fewer teeth, subtracting 10% from 33 you get 30.
Your numbers may be different but the percent rule is never wrong.

Or you can do what probably most of us do, which is drive thru a radar zone and determine our % error and after that,just do the mental computations for the various speed zones we encounter on a day to day basis. Ten percent is easy math lol.
Click to expand...
Okay so if I'm counting a round trip of 100 miles I would actually be getting 110 miles?
 
The closest tire diameter to yours in this chart is 26.1 which calls for a 30 tooth speedometer pinion gear with a 2.94;

transmission pinion chart.JPG
 
Just a reminder when buying a speedo gear...
a lot of the little plastic gears are going for big money, like over $100 in some cases.
I found if you know the gear tooth count you can go to web sites that sell Jeep parts and buy the same speedo pinions at 1/2 or more than 1/2 the cost. It seems Jeeps have used torque flights for years behind even the AMC motors in them. They use the same speedo gear that we do but for some reason they are 1/2 the price than say Classic or eBay. Ya just need to know the tooth count.

I needed a new gear for the Demon last year when switching out 3.23 gears to 3.91. Every where I looked for the correct tooth (I think it was a 40 tooth- I forget now) it was like $100-120. I went to a Jeep parts web page and bam! $32. Same chrysler part number in a Chrysler box and Part worked perfectly.
Good luck!
Joe
 
Since this is a fairly fresh thread, I thought I would jump in with a question to all you speedo gurus.
The car in question is a 72 Duster, 3 speed manual. We will pull the current speedo gear this weekend and figure out what is in it. I understand the tire diameter and gear ratio calculations to get the proper gear, I believe ours is pretty close when using the automatic radar signs around town.

My questions are (this may sound odd):
1. Is the gear for the 3 speed a short shaft or long shaft type? (I'll know when I pull it but thought I would ask).
2. If I want to throw my speedometer off by say 20 mph, so it reads 40 mph when I'm actually doing 60 mph, do I go up or down in tooth count?
 
The A-230 manual three speed uses the same long shaft pinion gears as the 833 4-speed as well as the 904 & 727 automatics.

(This may have changed around 1976-1978 sometime.)
 
My answer to that one would be why bother?

You did ask. ;-)
 
John_Lemon said:
Since this is a fairly fresh thread, I thought I would jump in with a question to all you speedo gurus.
The car in question is a 72 Duster, 3 speed manual. We will pull the current speedo gear this weekend and figure out what is in it. I understand the tire diameter and gear ratio calculations to get the proper gear, I believe ours is pretty close when using the automatic radar signs around town.

My questions are (this may sound odd):
1. Is the gear for the 3 speed a short shaft or long shaft type? (I'll know when I pull it but thought I would ask).
2. If I want to throw my speedometer off by say 20 mph, so it reads 40 mph when I'm actually doing 60 mph, do I go up or down in tooth count?
Click to expand...

I'm not sure why you would want to do this, I can understand reading 65 when you are really doing 60 might prevent unwanted speeding fines, but hey, it's your car! The answer to your Question 2 is: change to a gear with MORE teeth than the existing one.
 
Best way to avoid a speeding ticket is to not speed.

Intentionally being 'off' as your eyes perceive it will not beat a radar, any day of the week.

Especially after an internet post stating your intentions.
 
Alaskan_TA said:
Best way to avoid a speeding ticket is to not speed.

Intentionally being 'off' as your eyes perceive it will not beat a radar, any day of the week.

Especially after an internet post stating your intentions.
Click to expand...
What the hell fun is that?
As if anyone knows who I am or where I am, I don't think you really follow what I am doing but thanks for the response.

Thanks Nigel, I got off my butt and did the math shortly after I posted that response.
The 43 tooth showed up today.
 
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