[larrie]

Howdy Folks,
I have been digging in the forum archives for the correct speedo formula and found it, thanks to all who have provided this invaluable info.
What the younger guys/gals in this hobby don't realize is how much research in libraries, dealerships and auto shops it would take to learn this kind of info. I have had my 67 SS since 1976 and conducted this speedo research in depth back in the early 80's and NEVER found the answer. THANKS AGAIN!
Sorry for being so verbose ... I just had to say that!!
QUESTION?
I plugged my variables into the calculations for speedo gear ratios and found that I have to use my .7333 multiplier to get the correct ratio. If one does the math:
What I have currently for gears
Driven 36T ÷ drive 15T = 2.4 but I need 3.25 for my setup according to the chart. WHY THEN do you divide the multiplier .7333 to get the ratio ... that is instead of MULTIPLYING it by 2.4?? Since it is obviously reducing driven output by .7333
Why did I divide??? My speedo was close before I started with a new tranny I built with a 18t drive.... I did the math using the old #'s to make it work .. multiplying created weird ratios.
I don't understand the math here.
Always grateful for this forums generous giving of their knowledge.
Larrie

 

[Mike]

Post your info and whatever program/formula your using.

 

[JodysTransmissions]

You are making this more complicated than it has to be. Hopefully this will help you a bit.

36/15=2.4

2.4 = SGR (speedometer gear ratio)

You state you need a 3.25 SGR to calibrate your speedometer/odometer

You must X's the SGR by your present drive gear to find the proper driven gear.

15 x 3.25 = 48.75 or rounded up is 49 tooth driven gear and not available

14 x 3.25 = 45.50

13 x 3.25 = 42.25

And this formula works the other direction, driven gear / SGR:

36 / 3.25 = 11 tooth drive gear

 

[Mike]

Trying to find what your info is but no response.
GM used a ratio adapter with some rear gears so in case you missed it

Off the Chart or Can't Buy the Correct Gear

Ahh, back where we started. We'll have to use a ratio adapter and perhaps a speedo gear change as well to tune in the speedo. GM used #3932220 ratio adapter on many Chevelles. This part has a ratio of 0.7333/1 and retails for about $75 list from your friendly GM dealer. The effect of this adapter is the same as lowering the number of teeth on the speedo driven gear by 26.7%. For example, if you needed a 48 tooth gear for your THM400, none is available but you can use this adapter with a 35 tooth gear to get the same result.

These adapters were used by GM for high (numerically like 4.11 or 4.56) axle ratios where the "correct" gear was larger than the largest available. The adapter screws on the speedo connector on the tranny and the speedo cable attaches to the adapter.

Another GM ratio adapter is #368022 at $117 list. It has a ratio of 0.9444/1 and lowers the speedo output by 5.6%. *** above also offers several ratio adapters.

 

[larrie]

Mike ... Jody, Thanks very much for your input.
I followed the calculations found at http://www.chevelles.com/shop/speedo.html ... so:
diff gear ratio 4.10
X magic # 20.2
______________________
82.82
Divide 82.82
by tire diameter
25.5" P225/70/14 including the scrub radius flat of the loaded tire
___________________
SGR 3.25
Now I look up in the chart for the drive/driven combo I need. I have 2 speedo trans gear holders for 36-39 driven and 40-43 driven.
The chart calls for 12T drive and 39T driven ... I have never seen a 12T T400 drive gear. (been a licensed Trans Tech since 82) I looked up in my GM parts bible drive gears for T400 for 64-70 chevrolet, chevelle, camaro etc...
Only listing is 16T,17T, and 18T.
By writing this all out the answer has come to me!
Consider the drive gear a pinion in a diff and the driven as the crown, so...
With my SGR being 3.25:1 it will require 3.25 turns of the trans output shaft to obtain one full revolution of the speedo driven gear ... yes?
I have used my 36T driven and 15T drive which equals 2.4 SGR, by installing my .7333:1 adapter unit I turn my output shaft 3.25turns and I get one full revolution of my speedo driven.
By brain hurts a little now ... so do we agree that this thinking is correct?
Larrie

 

[Mike]

Would seem you need the ratio adapter that GM used for the 4.10's or a local equivalent.

 

[JodysTransmissions]

Larrie,

Do you currently have a 39 tooth driven and a 15 tooth drive gear installed in your TH400?

If so, your SGR is 2.6 and not 2.4: 39/15=2.6

Please confirm your gear combo........your ratio adaptor calculations will need it.

 

[larrie]

Hey Jody... Yes it is a 36/15 combo with the adapter/multiplier/overdrive what GM calls it. Thanks again for you help.

 

[rkd]

It's easier sometimes to just figure out the percentage it is off by and get gears that are different by the same amount.

Its also pretty easy to just calculate this from scratch vice trying to figure out someone else's numbers and which things they have combined into correction factors, sometimes.

Just did this exercise and my 66 now reads with in 2 mph at 60. Kinda like fixing some of these little things.

 

[Mike]

Ron,he's got 4.10's and there's no gear combo that works which is why GM used a ratio adapter as OEM.
excerpt from above:

These adapters were used by GM for high (numerically like 4.11 or 4.56) axle ratios where the "correct" gear was larger than the largest available.

 

[JodysTransmissions]

Since your speedometer/odometer is reading faster than your actual speed, you need to add teeth to the driven gear to slow it down. There are no TH400 speedo gear combinations to do a SRG of 3.25:1 so a ratio adaptor will be needed.

My formula:

correct SRG / existing SRG = ratio adaptor
3.25/2.6= 1.25:1

2.6 x 1.25 = 3.25

If you install a ratio adaptor of .733, it will speed up your speedo error reading by double of your actual reading you have now.

 

[JodysTransmissions]

Since your speedometer/odometer is reading faster than your actual speed, you need to add teeth to the driven gear to slow it down. There are no TH400 speedo gear combinations to do a SRG of 3.25:1 so a ratio adaptor will be needed.

My formula:

correct SRG / existing SRG = ratio adaptor
3.25/2.6= 1.25:1

2.6 x 1.25 = 3.25

If you install a ratio adaptor of .733, it will speed up your speedo error reading by double of your actual reading you have now.

As I re-read my posting, I wanted to add some info and explain some things better:

You can use a GM ratio adaptor such as a .7333 as long as it is a slow down adaptor. This will do what Mike was talking about but, only if the ratio adaptor is a slow down type and configured as a 1:.733 which means for every one input revolution there will be a .733 output revolution.

In Larrie's case this formula will work too as long as the Ratio Adaptor Shop knows Larrie needs a slow down adaptor.

Another formula:

existing SGR / needed SGR = ratio adaptor

2.6 / 3.25 = 1:0.80
3.25 x .80 = 2.6

Adaptor recap;

Both of these slow down ratio adaptors could be used for Larrie's transmission, it depends on the available gears the Ratio Adaptor Shop has.

1.25:1 = 1.25 input revs to 1 output rev
1:0.80 = 1 input rev to .80 output rev

Sorry for the confusion..........

 

[rodderscorner]

nice that you found what you where looking for..
but back in the day, I just went to the local dealer and they printed out the
speedo drive and driven gears with what reargear
I still have that print out..