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Old 06-25-2013, 08:36 PM
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Speedometer driven gear count

Hello,

I need some input as both me and my transmisssion shop are stumped on how to get my speedometer back to the correct speed.

I went through the formula below and came up with a driven gear number but when I put it in the speed did not change by much; then I took it to a local tranny shop (reputable) who did their own calculation and changed both the driven gear and drive gear but again, still off by 20%. The tranny shop will be reviewing but at this point they are stumped. They did indicate they only tested it in the city with a maximum speed of 45 which would have shown them it was close.

Here are the details; I'm hoping someone can point me in the correct direction.

* 31" tall tires; 3.73 rear end
* number of drive teeth on the tranny shaft = 15 (was 19, tranny shop lowered it to 15)
* my speed is too high by 21%

In my setup I had a 19 teeth drive and by the calculations a 45 tooth driven should have done it; it didn't and I was still off by 25%. The tranny shop dropped it to a 15 tooth drive and a 37 tooth driven and I'm still off by 20%; better, but not good enough.
I know every additional tooth drops the speed by 2.5% so theoretically going back to my 45 tooth unit with the new 15 tooth drive gear should make it correct. (note: I do have the proper housing units for 30's range driven and 40's range driven gears)

The formula I used is Rev/Mile=20168/Rear Tire Diameter (in inches) and
N(driven) = N(drive) x (Axle Ratio) x (Rev per Mile) /1001.

So, some questions:
1. if every tooth added drops speed by 2.5%, how does adding teeth to the drive gear affect that number?
2. There seems to be a ratio that increases as the difference is larger the faster I go, can someone explain how that could be? (for ex. at 50 on my speedometer I was doing 48, at 80 I was doing 75, at 120 I was going 100, at 140 it was 112).

At this point I will be retrieving my 45 driven gear from the tranny shop to see what difference that makes.

Thanks for any help.
Gary

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Old 06-26-2013, 06:13 AM
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I get 14/34. Be sure the axle ratio is what you think it is.
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Old 06-26-2013, 09:12 AM
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If your speed is off by 25%, then change your tooth count ratio by 25%. Ditto for 20% or any other figure.

The percentage change per tooth changes for every gear available. For example, going from 45 to 44 changes by 2.3% while going from 36 to 35 changes by 2.8%. Same for the drive gears. None of that really matters if you just adjust your tooth counts as noted in the above paragraph.
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Old 06-26-2013, 10:37 AM
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Be certain that not only is your Axle ratio correct, but that the speedometer itself isn't a little worn out. I went through this with a customer. I kept telling him the math was correct, and that the drive/driven gears I had chosen for him were a factory pairing as well. We even got the tire mfg to give me the true rolling diameter of the tire and checked his air pressure. To make a long story short; his speedometer was wore out.
Using the TCI calculator Speedometer Gear Calculator and Charts - TCIŽ Auto
you need a driven gear with a theoretical 36.36 teeth, assuming your figures are indeed correct.

Please understand that in order for the drive/driven system to work mechanically, there is some inaccuracy in the system. Driven gears in the middle of the range tend to be more accurate than the ones at the end of the range. In addition, the whole speedo, cable, drive/driven system seems to be most accurate in the 50-60mph range and accuracy falls off to some degree above and below that.
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Old 06-26-2013, 04:43 PM
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It sounds like the clockspring is weak.
A speedometer can read fast and still accurately register mileage.
An odometer will register 1 mile when 1000 revolutions of the input is achieved.
How fast it does that is the speed in Miles per hour.
If you can verify that the odometer is reading acuratley then you can determine that the speedo is or is not reading correctly.
If you Put a 1000 rpm motor on the speedo drive it should register 60 MPH and clock 1 mile in 60 seconds.
Getting a perfectly calibrated 1000 rpm motor can be a daunting task, but if you can find a 5,280 foot stretch of road you can verify the revolutions of the cable per mile...which should be 1000. If that is correct and your speed is not registering correctly then the spring in the speedo head is wacked.
Also to get the exact tire diameter , i use a dab of grease on the tire, then roll the car untill there are 2 spots on the floor, then measure the distance between the spots ( circumference) and divide by Pi (3.14) That gets er pretty close
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Old 06-28-2013, 12:07 AM
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Speedometer

Thanks You everyone for your help,

To answer a few questions:
1. I purchased a rear end refresh with a new auburn limited slip unit and a 3.73 upgrade so that is what I expect it to be (that's what the invoice says); however, just be sure I jacked up the rear end and measured it and it is a 3.73.
2. the speedometer is the original out of the truck (1984) so I am thinking it could very well be the problem; I replaced the speedo cable a while back but never the actual unit (not that easy to source these days). I am planning on replacing the entire dash with an autometer custom mockup next winter but for now I have to make do. LATech, I do have access to a road that long so I will be experimenting shortly.
3. I got the exact tire diameter from the goodyear website (I have goodyear tires) but I did not double check their numbers as it looked pretty much what I expect it to be.
3. the TCI calculator was what I was using all along to calculate my numbers (not sure what the transmission shop used).
4. I'm ok with some inaccuracy but 20% is a little much I agree though that it seems to be pretty much bang on in the 50 mph and below range; I am just confused as to how the system can get worse the higher it gets unless there is a ratio built into the system? I would have thought it was 1 to 1.
5. Cobalt327, thanks but I would rather, at this point, try to find a combination with my 15 gear setup if at all possible.

Thank You for all your input everyone!
Gary
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Old 06-28-2013, 04:09 AM
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Quote:
Originally Posted by ggevaert View Post
Cobalt327, thanks but I would rather, at this point, try to find a combination with my 15 gear setup if at all possible.
36.4 teeth/15 teeth. Rounded off to 36, obviously. Something else is going on here if it was so far off when you have tried combinations that were close.
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Old 07-14-2013, 02:01 PM
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Results

Thank You for all the help folks,

I experimented over the weekend by putting in a 45 tooth just to see what it would do; it 'should' have changed to becoming really far below the speed but it didn't, it did go the other way (from above to now below) and it was pretty close for some of the numbers. I then put in a 44 and that is actually pretty acurate for the low and high speeds, not so much for the standard 45 mph. In the end, the speedometer I think has gone bye bye, either through a weak spring or something else; it is a 1984 truck with original speedometer after all; and it was a farm truck so there is/was lots of dust that I cleaned up from behind the dash. I'm thinking some of it is still in there so I will look at cleaning up the speedometer. Are there kits out there to rebuild the spring? I would rather not spend the $150 that LMC truck wants for a new one.

I was using my GPS which I validated against out daily driver to get mph's.

My end result is that @ 35 mph it shows 30, @ 60 mph it shows 50, @ 75 it shows 75.
and you can see a noticable jump when it hits 65 where it jumps right to 70 thereby causing 75 to line up when it gets there.

So, question,
* Should I change the speedometer cable first (it's only 2 years old) before tackling the speedometer (LMC truck has on for $150) or should both be done?

Once that's done then I will be going back to the 36 that the tranny shop put in...

Thanks!

Last edited by ggevaert; 07-14-2013 at 02:08 PM.
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Old 07-14-2013, 05:16 PM
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I would try lubricating the shaft that supports the indicator needle. If it is dragging, the magnet in the rotating bell cant pull it far enough to register correctly. A weak spring would allow a higher speed at low speeds, where a dragging / dry bushing would register lower.
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Old 07-14-2013, 06:41 PM
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Lubricate

Thank You LATech, I will try that this weekend.
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Old 07-26-2013, 10:43 PM
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wrap up

Just to wrap this up.

I pulled the gauge cluster and speedometer and it was caked with red dirt (matches the farm it came from) so I ordered and installed a new speedometer unit. While I was at it I also installed a new speedo cable and now my speed is inline with what is expected. It is still off a few teeth (needs at 39 and not 36.4 as noted) but at least now I can get it dialed in.

Thank You to everyone who helped out!

Cheers.
Gary
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