The Stovebolt Site Forums The Shop Area Driveline Speedometer issue 1956 3100

MickeyP checking in haven't heard if you were able to get that ID of that axle yet. Hope all is well.

Thanks H, been busy with other things plus HUGE amount of rain here in north central Texas. I realized I didn't lube the new cable when I put it in. Got that done and got the teeth count off the driven speedo gear. . It has 20 teeth.Thanks for asking.

OK here is the number on the rear axel. KAS 137W81 H

KAS shows up as a
1978
8.875 ring gear with
3.07 Ratio

Made on May 17 in Warren, MI

The H is throwing me as that maybe a posi source but will research that more. its not in the 1978 posi list I have.

as long as they have not changed anything.

MikeyP and hcb3200, The below pic from this axle code page in the 1983 Chevy Master Parts Book shows the same info as mentioned by hcb3200 in your reply. It also shows that was listed as a STD (Standard) differential, not the P/TRK (Posi-Track) differential. Hopefully that gives you all the details you need.

GDADs51 you are right. I had checked the tire rotation and got a reverse rotation on the wheels. There are two things that are confusing. One is the test for ratio. With one wheel on the ground I get one complete rotation of the drive shaft with only 2/3 of one rotation of the tire. The other thing is what's causing the speedometer speed indication to be so wrong.
The H at the end is a different font type. Much smaller than the rest and not in line with the other numbers/letters.
As stated above the teeth count on the speedometer driven gear is 20.

Turn your tire one full rotation you should get 3.07 rotations of drive shaft

Your speed is off because 3.07 was not a stock gear ratio in the 50s for a 3100
And the drive gear and driven gear in the transmission speedo output do notmatch that gear ratio of rear end with the tire diameter you have.

Some guys with more knowledge of the transmission you have will have to comment on right gears you need

I've done the tire rotation a dozen times. I get the same thing every time.
With one tire setting on the ground and the elevated tire marker at the 6 o'clock position and the drive shaft marked at the 90 degree position so I can see it, I rotate the tire and before the tire has made one full rotation, actually about 2/3rds of a full turn the drive shaft has completed a full turn back around to it's starting position.
I understand about the ratios not matching but now on to how I fix the problem. Adapters, another transmission, change rear ends?

how many turns does the drive shaft do on 10 full Tire rotation.
do full turns on the tire and see what the shaft does. count drives shaft to full tire rotations.
do more than one turn

in the end you fix the issue a few ways.
if you can get a driven gear that works with current drive gear that matches what you need that is the easiest.
Second easiest is adapter.
third is drive gear and driven gear combo. (more effort to drop drive shaft trans etc)
then more effort more expensive.
Change rear end or the gears in the rear end.
or come up with perfect size tires (not really easy or many not be realistic)

here is another method that you can do and not even jack it up.

Tools needed
A flat surface
Masking tape
6 feet of string or more
Chalk (for marking)
Simple five steps
Park your rig on a flat surface.
Tape one end of the string to the bottom (6 o’clock) of the rear drive shaft. Using a clock method to determine location, bottom would be 6 o’clock, and the top would be 12 o’clock.
Mark the rear tire where it touches the ground.
Now push your rig forward so that the tire make one full rotation or two (divide sting count by two if you do two)
Look at string on the drive shaft.
The string will wrap around the drive shaft and the amount of wrap it has will tell you roughly what gear ratio you got.
Example:
If the string is wrapped around the drive shaft 3 times and a little more but not a 1/4 more then you have the 3.07 to 1 gear ratio
If the string is wrapped around the drive shaft 3 ½ times than you have the 3.50 to 1 gear ratio
If you got the string wrapped around 4 ¼ times than it will most likely be 4.11 to 1 gear ratio.

You are checking it wrong, this is how you check open differentials,

Like previously mentioned, if your car has an open differential (non-positraction), you will want to perform the check with one rear wheel on the ground. Again, mark the inside of the tire and a spot on the driveshaft with your paint marker. Now, rotate the suspended tire two revolutions and count the number of driveshaft turns. The reason to make two revolutions is the open differential actually rotates half as many times as a ‘posi,’ because of the design of the “open” gear operation

Spider gears make one tire turn twice as fast if other is not.

MickeyP, I think Jerry (Hotrod Lincoln) gives a better explanation of what needs to happen and why when checking the axle ratio this way.


On the corrective action, others will most likely chime in here soon. But for my $.02 worth, you will probably need to get an adapter mounted to the transmission speedo drive output to get your speedo reading more accurately.

Edit: hcb3200 and Joe H beat me to the post button!

Yea totally agree with the guys above. Thanks Joe totally forgot it was open. (all min are posi so i hardly think that way)
I think its a adapter or go the speedbox route is the fastest and easiest way to get there.
Because like the other guys have posted. The driven/drive gears are really limited for that transmission.
And the adapters need ratios to match up all of the above so its a lot of ratio, on ratio, on ratio calculations.
But will let the original poster decide which route he wants to go.

Going with the above earlier post of a 20 tooth steel gear (assuming driven gear but others can clear this up)
Then that would mean you need a 9.4 drive gear. which is not a valid value.
if your drive gear is a 9 then you need a 19 gear driven side.
so we need to know number of teeth on drive gear.
If anyone knows what the stock drive gear teeth are that would be outstanding.

Using what we have for info so far - 1956 3100 3 speed (not heavy duty or OD) with a 20 tooth steel driven gear the following is what I could find in the 1929-57 Chevy Master Parts books:

Parts Group 4.338 GEAR - Plug - Fitting - Retainer - Seal, Speedometer Driven: GM# 3845071 1955-57 1/2 TON (3-SPD.) (exc. hvy. duty) equipped with 6.50-16 or 6.70-15 wheels and tires (20 teeth-steel)

Parts Group 4.343 GEAR, Speedometer Drive: GM# 3845073 1955-57 1/2 TON (3-SPD.) (exc. hvy. duty, O/D) (7 teeth)

There was a 19 tooth steel driven gear listed as available for 1956-57 1/2 TON (3-SPD.) (exc. hvy. duty, O/D), but was listed with trucks equipped with wheel/tire combinations 7 x 17.5 or 6.00 x 18 which don't apply in this situation.

hcb3200, perhaps this will help with an answer to the inaccurate speedo reading and what ratio adapter may be needed to get things back in sync???

I am using this site. because i am not in mood to do the manual math
https://www.5speeds.com/calculators/SpeedoCalc.html ( i changed to a different site)
https://www.inchcalculator.com/icwidget/?wtdm=w&wa=speedometer_gear&wb=MTE1NzA5ODQ3NTU=

the two above want close to 15 but I think they are calculating tire squish and a more modern transmission.

site below uses basics

https://www.omnicalculator.com/everyday-life/speedometer-gear this site put 7 and 19 close

and assuming a 29.3 tire diameter. Let me know if this is correct for his tire size.

based on this math.
if he uses the 7 tooth drive gear Parts Group 4.343 GEAR, Speedometer Drive: GM# 3845073 1955-57 1/2 TON (3-SPD.) (exc. hvy. duty, O/D) (7 teeth)

and the 19 tooth driven gear it looks like this combination will work for his 3.07 rear end ratio.
I would expect the 19 tooth not to show up as correct for his tire because its assuming something other than a 3.07.

but the 7 drive to 19 driven to 3.07 to 29.3 seems to work in the calculator.

note:
after i did the math the i think if its 7 then you need a 14 to 15 driven . but 19 may be really close depending on tire size.

I would like to know he actual drive gear in it.

So, the bottom line... which of these online calculators do you think is the most accurate? So some of these online things, some don't give enough boxes for the stuff you feel you need to enter.

OK, made the adjustment recommended and with two turns and both tires off the ground I got about 3.25 +/- full turns on the driveshaft.

With an open rear differential, it's possible that you'll be WAY off. The only way that would be correct is if you can be sure that both tires are turning the same amount of turns. If you turn both wheels 2 complete turns, you should get 6.14 turns of the driveshaft (assuming a 3.07 gear ratio.) A 3.07 gear ratio means that the driveshaft turns 3.07 times for ONE turn of the ring gear.

I'd recommend you use the push the truck for one turn of the rear wheels and then count the driveshaft turn, using the string method above.

At this point, it might be quicker to remove the rear axle cover and look for the stamping on the ring gear that will indicate the gear ratio.

Ok MickeyP that would lean us all toward its a true open 3.07 just like the code says it is.
if you want to wait a little bit one of the guys may comment on the stock DRIVE gear in your trans.
This way saves you time trying to peak in there and count.
once that is confirmed we can give you all our options.

Peggy M. they have all the boxes in those online calculators to put in enough information. Some of them do basic math. Others do some additional calculations like tire squish diameter under load. Others make some "rounding" assumptions.
The are ALL good at doing ONE thing and that giving you the instant calculation based on input. So you don't have to keep doing all the multiplications.
That being said. The speed stuff will never be exact when using the mechanical stuff with tires. Even tires of the same description size are not exactly the same size for each manufacture. As in you could be off a 1/4 inch or more on diameter depending on brand make model. And we are not even talking about tire wear of it getting smaller as it goes over time.

SO we use all the tools to get us to some combinations that are close. one is not better than the other but using all the calculators gives us a really good guess. Then pick the gear closest to what we think will work.
Then the OP can test it and see how it goes.

Of course GPS driven gear like speedbox removes all that ratio matching because just needs the cable going to speed to be the right rpm based on 1000 REV per mile.
Same with the all in one GPS analog dial units. For those of us in the 55-59 truck range (like myself) those all in one units are a wee bit pricey now over 1000.00 dollars in most cases. You cannot cram that cheap round speedometer gauge in that triangle shaped hole in the 55-59 and make it look good.

I was talking about some other online stuff. It's good to know that ... Thanks for all this input in here.

Gdads51. Are there any other gear options for the transmission that would fit from parts books regardless of vehicle and options?

OK, tomorrow I'll pull the cover and have a look.
P.S. you can all just call me Mickey.

Guys, FWIW, Below is a screenshot taken form the 1956 Chevy Truck Data PDF that specifies the standard transmission speedometer gear set teeth counts and a list of the various ratio adapters. Someone can do the math to figure out which of the listed ratio adapters would provide the closest correction for Mickey's 3.07 current axle ratio. Also attached is a picture of a little later NOS 7-20 speedometer drive set so you can see what it's supposed to look like.

Mickey can you get us a actual diameter of your tire as well as close as you can to 8th inch just for fun

Also for education for the thread readers don't forget you cannot count the worm gear on the shaft from the hole looking in. You have to count from the shaft itself side.
See attached.

shortcut to answer if anyone is playing along and doing the math. why we don't think of these the easy way i don't know. but here it goes the short followed by long.

with the following assumptions
3.09 rear end
Stock 7 tooth drive gear
Stock 20 tooth driven gear
1000 (really 1001) rpm for a mile / mile per hour speedometer
Then the OP will need a 1.34 to 1 Steward Warner type of adapter. if they even made this ratio. I have seen a lot on ebay less than 1 and more than one 1.15 and a 1.20 ish.
1.34 is on the money But something close will be really close on the speed with 1000 RPMS per mile

The basic math.
Stock rear end ratio on the truck and how the speedo gears are set based on the parts and manuals is 4.11 and 7 and 20 and 1000 of course
He is running a 3.07 if everything is stock its as simple as 4.11 divided by 3.07 or 1.34 (1.339 rounded)
why i did not think of this moons and days and what 8 pages of post ago I don't know.
Probably because we needed to validate rear end and all the other items anyway in case someone had changed something.

The long math

5280 feet x 12 is 63360 inches divided by PI (3.14) 20128 divided by tire diameter 29 (for easy math) 696 Revolutions per mile.
696 x 3.07 or 2137 drive shaft rotations per mile.
2137 divided by 2.86 (the stock 7 drive and 20 driven ratio) or 747 RPM per mile to output to speedo which is why he is off.

1001 divided by 747 is 1.34 (1.33868 if using 1000) ratio needed to increase the cable back to 1001 rpms ( why 1001 say, well you need to full ration on each count so you add one but since its so miniscule in calculations most people just use 1000 to keep confusion down)

And therefore 1.34 to 1 gear ratio adapter that fits the speedometer cables.

FYI the 4.11 does match the math calculations as well with the 7 and 20 teeth gears when doing the math as well with stock items. and 29 tires to validate my theory.

As long as the tires are close to 29 inches in diameter of course. change the tire size and have to change the adapter or something else of course.


One Note the actual driven gear teeth would be 15 with all the items above if you were to replace the stock 20 gear to get to the correct ratio without a adapter. But I don't believe a 15 gear is available for this transmission and speedo type.

OK here is the number off the ring gear. 11 779 14:43 5 781 numbers weren't clearly stamped. Bottom half of first 2 numbers (11) missing and the 81 at the end a bit unclear. Actually the clearest part is the 14:43. So...
Looks like we have a 3.07.
The rear end has a howl at low speeds so let's assume I need to overhaul the rear. Can I get my speedometer closer by changing out the gears in the rear without wiping out the highway performance?

You could change any of the parameters (rear end gear, tires, speedo gears adapters etc) just depends on what you want to do long run on the build as in are you keeping stock period correct tire sizes. and your budget.

You could do a 3.50 or 3.73 rear end gear change out.
if you do it just changes the needs of the speedo gears.
going from a 15 now to a 17 tooth or 18 tooth respectively.
So in the end you are changing something on the gear speedos unless you get to a 3.90 or 4.11 rear end which is going the opposite of where you want to go.

if your desire is to keep the good highway and milage performance.
rebuild or repair your rear end. Leave it at 3.07 and get the gear adapter 1.30 (three on ebay at moment) and you will be closest.

And the 1.3 gear adapter will work with the existing driver gear? Another question. I just put the cover back on the differential and when refilling it at about 3 pints it started to run back out. The manual says 5 pints. The truck was jacked up to pretty near level. My garage floor, of course has a downward slope towards the overhead door. I plan on driving to local garage and having it checked as soon as I get it back on the road.
What do I Google to find the ebay adapter?

The adapter is a Stewart Warner 777. 1.30. There are a few flavors of 777 so make sure the right ends are on it.
I think this is closest. You need a 1.339 or 1.34 based on the math. Speedometer adapter. It will screw in where your cable now attaches fo transmission. Then you put cable on the adapter. Again You need to verify the fitting ends in case it’s not labeled correctly by reseller
These were made for a lot of different car manufacturers but it’s all same principle.

Just check the level again in morning it may not have settled in all locations yet.

Note there are some speedometer shops that will custom make the ratio. But they want a bit more money than the eBay already made ones. I would go with ready made then you can decide if it’s close enough or if you more refined. You can always resell the other one if you go custom

This may be your perfect one.

It’s 1.339. And you need that 1.34

https://www.ebay.com/itm/116073385184

That one was already out of stock but I found this one.

https://www.ebay.com/itm/126382383336

Note: this one appears to be out of stock also.

The 3.07 will give you the best performance on the highway, but will limit your hauling capability unless you change your transmission to a T5 and get the proper ratio chain. The 12 bolt rear end (like many others) had limitations on the gear ratios, so in your range (as I recall) you would be able to change to a 3.42, but possibly not to the 3.73.

Actually Jon I don't plan on doing any hauling or pulling with it at all. I have a F-150 to do that. Thank you for your help.

Although not the best match as found by hcb3200, here is a 777F 1.30 ratio adapter that is still showing as available. There are a number of speedometer and cable companies advertising on the web for ratio adapters. You may want to reach out to one or more of them to see what they offer.

Thank you. I already ordered the 1.339. Hcb3200 said that a 1.34 is exactly what I need so the 1.339 should do the trick. I was amazed at how many different adapters are still around. The one I bought is a NOS that the seller changed the ratio on to get the 1.339

To test you can turn it on drive side and output should turn a full turn plus just past 4pm spot. 122 degrees

Do not pack with grease or lube. I found old steward Warner manual that said if any lubrication it was just a dip in 10 weight oil.

I've been wondering how it would get lube. I'm going to ask the seller if it needs to be lubed.........

Seller says it was lubed and is completely ready to install and use.