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

New guy here so be gentle. Sold my '55 convertible and going through withdrawal symptoms caused me to pick up a pretty nice '56 3100 three speed 6 cyl. There are some things that need fixing and the first thing I tackled was a speedometer issue. Driving down the road the needle continuously bounced back and forth between about 15 and 35 mph no matter my speed. The first thing I've done is replace the cable and the needle no longer bounces but stays pretty stable. Problem now is that the speed is wrong. Speedo shows 20 and I have to be going about 40 or more. Anyone know how many teeth the little gear the cable fits in has?

You care more about the gear in the transmission side. The speedometer is fix at 1000 revolutions per mile.
So you adjust the gear in the sender to match the ratios that take into account the tire sizes, rear end gearing and revolutions of the shaft to that number.
There are some apps that allow you to input the data.
Or you can post what transmission your have. What is your tire size and what is your rear end gear ratio.

The bouncing would have definitely been a busted cable or binding which you fixed with replacement.

https://www.stovebolt.com/techtips/driveline/calculate-rpm-tire-axle.htm


go to the link above it will have a tech tip on what you are asking.

If you pull the sender you can look in trans to see what gear color is as you will have two gears that makeup the ratio.
A drive gear (one in trans on shaft) and driven gear (one on sender your speedo cable is connected to).
Check out the colors it tells you what the ratio setup is.

Here is a example from the 700R4 trans to get you started.
Until we know the rear end gear ratio and transmission type / sender type. We can advise on what ratio you need.

Here is a site to input all those values and it will let you know what you need

https://transmissioncenter.net/shop/speedometer-gear-calculator/

It could be your speedometer head. Try this: remove the connection from your transmission and connect the cable end to a cordless drill. Then spin it...very slowly at first. See if it rises to around 60 or 70 steadily as you increase the drill's speed and then smoothly drops when you reduce speed. If it does that, it is not likely you have a problem in the head. If on the other hand it rises sluggishly and then seems to reach a point where it doesn't rise much anymore, you might want to remove the speedometer and do some checking. On the back (where the cable attaches) you'll have a 2 part brass affair. Please see image one below. If you move this between your thumb and forefinger you should see movement of the needle rather quickly and this end piece should spin very easily. If you flick it like you're snapping your fingers, the needle should jump to around 30 mph quickly and then drop back down. If this feels sluggish, remove the brass plug (second image), add a light oil (like 3 in 1 or sewing machine oil) and work it until it loosens. You can also lubricate where the 2 part brass piece is split by a thin washer. This all needs to be very smooth. There is also another place where you can lubricate...this controls your odometer shaft, but it usually is okay. In the final image you can see the other oil location. Please ignore the red arrow. That image was used when we were repairing a broken needle shaft here a couple of years ago.

Thanks all but remember I'm new to the 3100. The tires are 6.00 x 16. I assume it's a standard three speed manual trans and rear end. I do know how to determine the rear gear ratio and will do that. The small gear that come out of the trans is metal and thus has no "color". I can pull it again and count the teeth. Is there a way to determine the needed information on the transmission?

I second what Jon G is saying, I cleaned and lubricated the speedometer and it made a nice improvement.

Also get the actual transmission model. Not just 3 speed manual post cast numbers and the guys here can figure out what you have. Pictures help greatly.

Mickey, here is another thing to try...have your wife, friend, etc drive beside you at only around 10 mph in a different vehicle. See if the two speeds agree. If they do or if they're close at that low speed and if your truck fails to increase indicated speedometer speed as you accelerate, look to your speedometer head. I won't say it is a guarantee but let's see what you discover. Also...fairly important...if your speedometer fails to agree with actual speed by 50% or so, let's try to figure out what ratio you have in the rear end. There are some easy ways to do that but first let's see what we see.

6.00 x 16 tires are going to be 28 inches in diameter. All 3 speed transmissions end in 1:1 ratio (3rd gear). Just for sake of calculation here.

The easiest thing to do is open Google Maps on your phone. Enter a destination and start driving. It shows your actual speed as you drive. I recently used this for my Galaxie 500 and discovered that the speed and miles are registering exactly 12.5 % low at all speeds.

Mickey, Welcome to the Stovebolt. I use a very accurate phone ap called Speedbox (gps powered)to check the speedometers of my various rigs.

OK here's where I am. I put in a new cable and that stopped the bouncing back and forth between 15 and 35. The needle still vibrated a little and the speed is is definitely wrong. Today I put in a new speedometer. It still doesn't budge until speed is about 20-25 but the needle operates smoothly and stays stable. Sure looking like a mixed ratio problem. I can pull the small gear and count the teeth. I'll also check the rear end to confirm ration. Can someone tell me where to look on the transmission for a model number. I'll see if I can get pictures. I'm assuming the instrument cluster is correct for a '56. It has 4 guages rather than the 2 that the 55 has? Or does it have 4 because it's a truck? Yes, I do have maintenance manuals on the way. I really appreciate all the help.

The 55 and 1/2 thru 59 all had same triangle shaped cluster with 4 across the top with the speedo in the middle. the lower half of 55 (they split the year with two different models) had the same gauges as previous years.
So I am sure its correct. That rear end ratio will help because there are couple different ratios you see on these trucks. and this stuff happens if it has been replaced or the trans has. Since you said your gear teeth on trans side was metal, both the driven and drive gears are matched sets. you cannot change one without changing the other normally in that style. but will know more when you get us more details on the trans. All mine have been automatics so some of the other guys will have to help you out on where to look on the manual ones.

The number on the side of mine is: 3845122. Date code. B 2 7

Fred I'm trying to find out why my speedo shows about half the speed I'm actually going (estimated) and how can I fix it. I just checked the rear end and something doesn't seem right. I did 2/3 of a tire rotation for the drive shaft to make exactly 1 rotation. The casting numbers on the rear end are 3896839 with a "N" below that. The other side a MM 86.

Is this by any chance a 12 bolt rear end?

You need to make sure the other tire is stopped and try again.

Thanks guys. Yes it's a 12 bolt and one tire is resting on the floor. I've done it over and over. Still get the 2/3 tire rotation to 1 rotation of the driveshaft.

I marked the tire at 6 o-clock and the driveshaft at about 90 degrees to center of rotation. Slowly rotated the tire and watched the mark on the driveshaft. When the drive shaft came back to the 90* position the tire mark was at about 2 o-clock. The other tire was sitting on the floor.

Try a full rotation of the tire and count driveshaft rotations instead. Might be a little harder to determine partial turns of the driveshaft, but you'll get closer to the actual ratio that way. Or use multiple tire rotations till you get full rotations of the driveshaft. Then divide driveshaft rotations by tire rotations to get twice the ring and pinion ratio.

Thanks, I did that too. One full rotation of the tire gave 1 1/2 +/- of the driveshaft.

MickeyP, There should be some code letters/numbers stamped into the 12 bolt axle passenger side axle tube. Those will help identify what the differential ratio is in the axle as well as what the axle originally came from. Usually a 3-letter code is what is used to ID the ratio and the other data is for original application and where/when manufactured. See what you can find and post here so we can try and provide further help. FYI - from your rotation count, you most likely have a ratio in the 2.56-2.73 to 1 range. That coupled with your 1957 transmission 1:1 output would likely cause the misread at the speedometer as others mentioned before the original rear ratio would have been 3.90 or 4.11 to 1. More info when we hear back on your axle details.

Here is what I found. 3896839 left side of rear with a "N" below it. The right side had GM 86. There may have been maybe a 2 digit something but I couldn't make it out . I'll look again. I did check the tubes but not closely and didn't find anything. I'll also check it more closely.

Below is a screenshot from another source that shows where to find the stampings on the passenger side axle tube. Top of the tube, of course in the worst/hardest to reach and see spot.

Look for 2 letters (unless yours is after 1970 as I recall). Mine is RA. Here is what you'll probably find if you have a truck rear end:

HB,LL,RA.TR,TT,TU are 3.07

HA,HJ,HQ,MF,TQ,MI,MK are 3.73

HK,HL,HR,ME,MG,TS are 4.11

HC,HN,MU are 3.73 Posi

HO,HP,JQ, are 4.11 Posi

If you have a passenger car rear, please tell us and we may be able to ID it also.

You guys are determined to keep this old guy squirming around under that truck.

Squirming around is the fun part. Nice looking truck by the way, Mickey. If you get really frustrated and think you need to sell it, please tell me.

J glad to meet a fellow Texan. I just sold my 55 chevy convertible and shipped it to Minnesota. Then saw this 3100 and had it shipped here from Idaho so I guess I'll stick with it a while longer. At least it runs and drives really well. That is if you don't care how fast you are going. J, even if we figure out the rear end ration and the same for the transmission. What is the solution going to be? Is there a fix? I do have a spare posi rearend that I pulled out of the convertible. Think it will fit. I know it's a 3.73.

Mickey, I was born in Lubbock and grew up down near the Mexican border (Big Bend Park area), so yes...a Texan, I'd say. Or Texican, some might say.

Sure, there is always a fix for something like that. Once we know about the rear end ratio we can see if the speedometer gear can be made to match it. There were limitations on those gears, however. But that aside some folks sell converters that will let you step up/step down (I should add I'm not a fan of them, but they do work okay). And if everything else fails I've made a GPS "look-alike" for several models of the AD truck which is smooth and accurate. The only negative is you can't have an analog odometer with it...but people who have seen mine so far haven't noticed that. Let me hunt down a link to that post for you. With my truck the T5 transmission I used was set up for electronic speedometer so a cable drive was going to cost more and be costly.

Growing up a friend had a 1956...well his dad who owned the Gulf station had it. We drove it all over the State (even over into Mexico back when that was okay).

Let me find that GPS post for you.

See if this link helps:
https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1453840/1.html

Can do same result as Otto, mark mileage at some mm, and note the difference at mm plus 10 or 50 etc. miles, that is the % your speedo is off.

Ed

I took a closer look at the axel and can find no numbers. The axel looks like it has maybe been sanded and painted.l did find another number on the barrel, E158 and near it another location the number 96 or 69. Hard to tell.

where did you see those number?
normally the ID with the gear code, day of year manufacture, posi type if equipped is all stamped (not cast ) in this location
The stamped number we are looking for is stamped on the forward facing passenger side axle tube and It'll usually be about 3 inches away from the c

The E128 is most likely
Manufactured on May 15th of a year ending in 8
GM86 on it somewhere as will that would most likely mean GM 8.6 inch with the 12 bolt cover as seen in your pictures.


The stamped number we are looking for is stamped on the forward facing passenger side axle tube and It'll usually be about 3 inches away from the carrier housing (center section of rear end housing).
Will look like this
Rear Axle Code example: CB G 112 1 E (the E here is not the same as the E above due to formatting tells us what the stamp means)

if it is somewhat modern. Since that axle is painted i am sure it is covering some numbers.

if you are not in the mood to grind your paint and find it. try a pencil rub first. or if you take a nice in focus picture with your phone of this area sometimes zooming in will show what the eye cannot see.
Otherwise its going to mean a more precise tire rotation and axle turn count to get to your number.

Once we have those first two digits we can get that ratio the last missing piece to determine what you can and cannot do with the stuff you have.

Details if this helps

this stamp can be difficult to locate on an 30+ year old axle due to layers of paint, undercoating, grease, and dirt. It may be necessary to partially strip the forward face of the passenger side axle over the middle third of the length in order to locate and discern the characters. A razor blade and/or putty knife, fine wire brush, and cleaning solution may be needed tools in this search.

Rear Axle Code example: CB G 112 1 E
(CB = Ratio, G = Plant, 112 = 112th day of year, 1 = shift, E = Positraction Source)

Assembly Plant Codes
Buick - B
Buffalo - C
Cadillac - D
Detriot Gear & Axle - G
GM of Canada - K
Warren, MI - W
Oldsmobile - O
Pontiac - P
Pontiac/Canada - M

Shift: D or 1 = Day shift| N or 2 = Night shift

Positraction Source/Code:
- = No positraction
D = Dana Positraction Differential
E = Eaton Positraction Differential
G = Chevrolet G&A Positraction Carrier
O = Oldsmobile Positraction Carrier
W = Warren/Warner Motive

Positraction Casting Numbers
Due to the possible confusion with gear ratio descriptions... a HIGHER number is a LOWER gear.
In the below examples, 2.56:1 & down = 2.41, 2.56, etc... which is actually a HIGHER gear ratio.

8.5" "Corporate"
Casting # Series Gearing
410408N 3 2.73:1 & up
410409N 2 2.56:1 & down

rear axle casting date code Rear Housing Casting Date

Example: D143 - (D = Month, 14 = Day, 1 = Last digit of year (1973)).

Month: A = Jan, B = Feb, C = Mar, D = Apr, E = May, F = Jun G = July, H = Aug, I = Sep, J = Oct, K = Nov, L = Dec

What I'm hoping to hear is either a 2 letter or 3 letter code which is stamped into the axle tube on the passenger side and it will be on top of the tube. Your axle would have to be sanded a lot to obliterate that stamped in code. Probably possible to sand it off but unlikely. You have a truck rear end and it could have been made between 1962 and about 1980. The code is not always easy to see but it should be there. As I mentioned, mine's RA and that means 3.07 ratio. Look carefully under light. Two digit ratio codes were used up to a point (think this may have been around 1970, maybe 1972) and then the ratio was identified with 3 digit codes.

OK, so top of passenger side axel about the middle third. Now I assume that means from the side of the center section to the back plate on the wheel.
I'll see what I can find.

You are looking in the area in the picture attached either on top or on on driveshaft yoke side but it will be on the passenger side for sure

Bingo. I was feeling around with my finger nail and I believe it's there. I'll find out for sure tomorrow after church and after a granddaughters birthday. You know, first things first.

ok most important is the first 2 (or 3) letters. And they will be letters.
the rest is nice to have but to solve your speed issues that tells us the most.

the last letter if there is one. Will tell us if it has posi traction. (and what model type)

Well it's not a posi so don't have to worry about that last letter. Do we?

Nope. Not for your speed issue.

If it is not a posi then the last letter is silent...and many simply won't understand that.