The Stovebolt Site Forums The Shop Area General Truck Talk 55.2: Odometer = Accurate ; Speedometer = Not accurate

From a different thread- 1955 2nd 3100; 3 spd; 215/70/15 tires; 12V--- The truck's speedometer reading is not accurate and I would like to know options to get it to show an "accurate" reading.

The odometer functions and is correct. This is verified by known distance driving and also by a lead vehicle (pace car) readout/comparison.

The speedometer reads 12 MPH at a known speed of 30 MPH, and the speedometer reads 28-30 MPH at a known speed of 45 MPH. The speedometer does not move at all until I get to 2nd gear speed (or if I am driving too fast to be in first gear; I checked). The speedometer needle doesn't flop around or act erratic. I replaced the speedometer driven gear (inserted in transmission) with the exact tooth count plastic reproduction, however, I do not know if the original ever functioned. The speedo cable was severed when I got the truck.

Thanks for any guidance; worst case scenario, as of now, is I'll just know 30 MPH readout equals 45 MPH. Of course I do prefer the thing to be accurate.

Brad

Edit (Chief has made me look for details, lol)- a new speedometer cable is attached now, new and/or rebuilt gauges in the cluster, and as mentioned, I connected the speedo cable.

Jim Carter sells a ratio adapter - in attaches to the speedometer cable fitting in the transmission.

Well now, let's see, new speedo gauge, new cable, "new gear"? So....assuming all that is really true and all are correct parts...something is not right, right? Or something is slipping. Don't think that the tires would cause that much. Are tranny and gauge original to truck? The exact tooth count gear for what trans? exactly like what? the old one? What was wrong with the old one? You and I probably suspect the wrong gear, slipping gears, speedo not rebuilt properly, cable square slipping in the female. If everything is "engaged" properly, then it's a ratio problem as Tim indicates. Because the speedo IS being driven. My guess: bad speedo gauge because I know you made sure the cable was engaged properly on both ends. BUT I don't know what trans, from what year, from what model vehicle you have. The fact that the odometer is correct and speed is not means it's the Speedo gauge provided you drove far enough to pick up any mileage error.

If the odometer is reading correctly I don't see how it could be a "ratio problem". Sounds more like a speedometer mechanism problem. Maybe weak magnets. Maybe needs lube.

Agreed.

I could not figure out a mathematical ratio 'answer' to the speed difference...

55.2 235 (original/rebuilt) engine; 3 speed transmission (original); new plastic replacement speedo driven gear (old metal one had broken square-end cable inside- not removable); new speedometer cable; new speedometer gauge (or, purchased as a "new" one). For sake of argument, trust that when I have to use "new" as an adjective, that was the only option.

The cable was engaged at both ends & engaged properly. At first I thought about chucking the speedometer cable (at the trans end) in a drill to test it, but it was just as easy to drive the truck, and I didn't want to figure out which direction the drill needed to spin.

I drove five (5) odometer miles to let the new speedo parts "meet" each other & for us to pick out our 30 MPH & 45 MPH checkpoints (CP) on the route; I dropped my friend off at his truck & followed him three (3) miles out, and three (3) miles back noting the speed at our two CP each way (0 MPH start, 30 MPH @ CP 1, 45 MPH @ CP 2). The speedometer was consistent, registering the same incremented values each way. The odometer was correct within 1/10th of one mile at the turn around point, and was correct upon return to base (or, it was the same as my friend's digital readout/trip counter in his truck).

I will look for some pictures.
Thanks for the help.

Brad

Added- I have verified the engine is original to the truck.
The only family member who ever did any wrenching on the truck said he only disassembled, checked, cleaned, & reassembled the transmission (mid 1990's).

I am calling the transmission "original" based on the fact that nobody in the family ever remembers it being replaced. And everybody remembers something about it.

Brad

edit- So, the transmission is a 1955 3 speed transmission on a 1955 2nd Series 3104 Chevrolet. edit 2- no, I don't know the model number, but it weighs about the same amount as a MK-19 receiver. picture of it from a while back. You can almost see the broken cable in the original speedo gear in this picture.

Does anyone remember that the speedo was once accurate? It does still seem like you got a bad speedo. Who or whom did you get it from? A reputable vendor with a guarantee, or a guy with a short cigar at the swap meet?
People do say run them with a drill but you already know it registers. Nobody knows what rpm on the drill would be what MPH on the gauge so it's useless to check for accuracy. Are all of the mating gears in the tranny steel except the replacement? What is the part no./vendor for the replacement gear you bought?

Consider this. If your needle is on incorrectly such that the speedometer can not return to the stop, that would explain why is doesn't move until you are up in speed. It appears you may have an offset error of -18 mph. The needle being on wrong fits in with the offset error too.

Maybe with Tim's comment in mind, you can discern if the needle is dropping past the first hash mark/DOT or stopping higher than it at rest.

This is from the web:

"Very carefully set the needle on the shaft at about the second long bar up from
the lower left position. Before pushing it all the way on, turn the shaft from
the back to verify it moves up when you turn the shaft. (If it doesn't, you are
turning the wrong way or you missed a step somewhere. Back up until it
works.) As you push on the needle from the center, carefully twist left until
the needle lines up with the first long white line on the face. Keep spinning
the shaft after every adjustment just to make sure it drops back to the line.
You can now make sure the needle doesn't hit the face by carefully moving
the needle by pushing on the shorter bottom part. If it touches the face
anywhere you can gently lift on the end of the needle with the blade of a flat
screwdriver. JUST DON"T BREAK IT OFF. I don't know ifthey are
reproduced or not.
You should now have a silent speedometer when reinstalled. Don't forget to
lube the cable before you attach it to the back of the head."

Back in the 60's Aunt Maudette calibrated the speedometer with her GPS... (True name)

No sir, I have never heard anybody talk about how amazing the speedometer was. I purchased an entire cluster from an eBay member ( aresalesaz ), who bought it from a reputable rebuilder on eBay. There was a lot of communication prior to this purchase to make sure the "rebuilt" actually meant "rebuilt" with all new components. I also back-checked & fact-checked, and sure enough they did purchase a rebuilt cluster from this reputable rebuilder. The cluster's oil pressure gauge failed during my engine run-in almost causing me a heart attack, I can't get the fuel gauge to work, and now the speedometer (which I am still going to troubleshoot). I am checking the engine temp with an IR thermometer, but this gauge in the cluster seems to be OK. This aresalsaz seller did not have to respond (and does not) to my question since the transaction was older than 90 days. The cluster, complete & pretty was going to be cheaper on me than the parts & gauges to rebuild my old one, but not so cheap as to arouse any suspicion. The old cluster had mechanical temp gauge, but you all (about the same group now) talked me through what went where, etc on the new cluster.

I am sure the reputable rebuilder does fine work, and I prefer not to mention them. (edit- I just got a little ticked off about remembering the oil gauge situation- not fun at all)

Brad

truckernix- I'll look at that, thx
Chief - ditto
cletis- thanks & what type lube goes on the cable? lithium grease?
Tim- I hope that adapter isn't required, but thanks

Here's the only pictures I have of the gauge clusters.

And, I am not ruling out operator error on the fuel gauge- this thread is specifically on the speedometer.

Brad

Super nice looking cluster. Looks like someone was careful. I've been careful and screwed things up anyway. The worst that can happen is you have to buy a speedo unit and drop it in the cluster. Not the end of the world. Actually the worst that can happen is you did that and it wasn't the gauge or the replacement doesn't work either. Remember? I am a professional life motivator. A positive mental attitude proponent.
Chief of positive thinking Human Beings.
Maybe you should write a nice email to the actual builder and explain and ask for any suggestions or tests or observations you could do. Probably a nice guy and he would help. Unlike myself.

Brad,

My bet now is that an adapter is not required.

Brad, I have some 55-56 Chevy car clusters. If someone can verify the speedo unit is the same as truck, I will disassemble and send the unit to you to try.

Thanks! I have the old one off my truck's old cluster I can tinker with first. Both clusters registered 0 mph when I photographed them, so I assumed they were calibrated.
Brad

The magnet for the speedo can loose its magnetism. What did happen to me was my odometer was not working but speedo was correct. Took the cluster to the local reputable speedometer shop, they fixed the odo, but somehow "lost" the magnetism for the speedo needle and they were not able to re-magnetize it. Result: new aftermarket gauge mechanism...
Seems to be your issue, as explained in the other thread you quote at first. It seems difficult to re-magnetize the speedo magnet.

Will look into this, thanks.
Garden tiller needed repair, so I'll get to the speedometer as soon as possible. Much appreciated , gents.
Brad

What type of lube is good for the speedo cable shaft (at the cluster connection point)? edit- (I forgot I can use the search on this pc, and read several posts) I can remove the entre speedo cable if this is a better option.
Thanks
Brad

I wouldn't suggest any lube at the cable/speedo head connection, just need to make sure the speedo unit is quite clean inside [tech tip for TF speedos] - the cable itself should also be cleaned, inner and outer, and a bit of dry graphite wiped on the inner when it's re-inserted

Bill

Brad, I use Lubriplate white grease on the cable- - - -sparingly, a little goes a long way.
Jerry

I haven't got back to the speedometer just yet, and we can't remember if I did or did not rub white grease on the cable (I had white lithium grease out last week and don't know if it would be the same?). Maybe tomorrow I'll get to checking out the speedo.

We'll just say the burb's going to keep its speedometer for now, and thank you for offering.

Brad

The speed is indicated by a rubber cup cone assembly. The faster it turns the further out the cup travels and that moves the indicator that translates to speed.

Miles travelled is strictly gear driven. X amount of turns equals x miles travelled. The odometer is correct but the mph is off is a common malady reported on the older BMW motorcycles and that is how it was explained to me. Calibration is performed by adjusting the position of the cup on the cone, that's why it doesn't improve when you replace the speedo cable, since the speedometer is still the same assembly.

I have not heard this before... I will check it out- thanks.
Brad

Brad, see the pics in this tech tip
the speed indication is magnetically driven by a spinning metal bell against a coiled spring, no rubber involved and it is "calibrated" by the driving gear in the trans and driven gear on the cable .... no old Bavarian tech in GM trucks

Bill

edit - I'll add that it sounds like your problem is the needle is "misplaced" on the pin, needs to be [carefully] moved up so it starts moving as soon as the vehicle starts moving

Thanks for the confirmation, Bill.
Brad

It's not really old tech, just a poor design. I had the same problem on my brand new 2004 RT.

How do you calibrate the speedometer unless there is an adjustment between the bell and the spring? Doesn't the odometer being correct indicate he has the correct gears?

BTW, Thanks I love learning new things and this site is great.

no calibration possible or needed [other than correcting the needle position] if things are as designed - the original engineers calibrated the gearing, magnetic strength, return spring and number template to be 'reasonably' accurate - if the reading is very far off, something needs replacing .... "calibration" for old trucks would involve measured miles and stop watches and making note of any error, most likely constant over the full range, same as with any of the gauges, none of them are of scientific accuracy

Bill

so were these shops that I remembered from a my high school days that advertised speedometer calibration just making stuff up?

"Calibration" in terms of new parts, cables, cleaning or "needling". As Bill indicates. Not in terms of turning a screw or using an oscilloscope. They made it read correctly for $59.95 plus tax. The eddy is the eddy and they probably made any major corrections at the tranny end with a tooth change.(that would be extra)

Thanks for the info

I'm glad you brought it up danzimbric. My odometer is still reading just fine, and I will update whenever I get back to the speedo problem. The truck is running & driving fine with me not knowing exactly how fast I'm going.

Brad

I "calibrated" mine using a electric drill motor. I had the speedometer out for a cleaning and new decals, so I figured it was a good time to check it. I have a hand held, mechanical rpm gauge I used to set the drill rpm with. Most all speedometers were set to run 1000 rpm (cable revolutions) at 60 miles per hour. I set the drill to run a 1000 rpm and set the throttle stop to always come back to that speed. I then installed the speedometer and ran it for 10 miles watching the odometer. Since the odometer is a direct gear to gear, it will read accurately to what ever rpm it is being run at. Once I did a minute rpm adjustment to get 10 miles in 10 minutes, I then started looking at the speedometer needle. It took a couple of tries, but all I did was remove the needle and reposition it till it read 60 miles per hour with the drill running at 1 mile per minute. At every other speed, the needle is slightly off, and the faster it goes, the slower it reads. 60 is where most of my driving is anyway. At slower speeds like 35, it's relatively close and I can judge from other cars how fast I am driving.

When I put it back into the dash, all I needed was a simple transmission gear change to get the odometer correct. Knowing the speedometer head was reading right, changing the trans gears was really simple. Now it tracks right with the highway markers.

It's not a true calibration, only a shop with special magnetic tools and new springs can do that, but for the DIY'er, it worked really good.

If you decide to try this, be sure to have the drill and speedometer setting at the same angle as the dash. The weight of the needle will change the reading.
Joe