Speedo Reading Corrections?

[55 Fargo]

The speedo gear from the M5 semi-auto trans is not the same size as the 3spd in my car, it's too long, so I will stick with what I got, maybe get an adaptor to correct this some day..........Fred

[De Soto Frank]

Fred,

For what it's worth, in my '48 New Yorker, with bone stock running gear, and CORRECT bias tires (8.00 x 15 ) , my speedo registered about 10 MPH "slow" at 60 MPH actual.

Since I have a 50 mile round-trip to/from work everyday, 20 miles of which is on I-81, I usually check my speedos against the mile posts/wristwatch.

Once I get the vehicle moving at about 1 mile /minute (60 MPH), I then try to maintain that speed over several miles, to minimize chance of error.

Once I establish a "baseline" error factor at 60 MPH actual vehicle speed, I guestimate from there.

The '41 De Soto speedo is about 5 MPH "slow" @ 60 MPH actual, my '60 Windsor is about 7-10 MPH "fast" @ 60 MPH actual (but I'm running radial tires that are "shorter" than the OEM bias-plys. The odometer in the Windsor also "gains" about 1/10 mile for every actual mile covered.

Don't know how convenient nearest marked-mile is for you, but if you can, timing over a known measured distance would give you the most accurate accounting of your vehicle speed.

But hey - you're driving it now !

[Normspeed]

Fred, you could use the dual purpose old Mopar speedometer and fuel guage: Lick your left index finger and hold it out the window in the wind as you drive. If a cop stops you, you're speeding. If the car quits, you're out of gas.

Australians and New Zealanders, use your right index finger.

[55 Fargo]

Here is another thought, the engine trans is from a 1951 Dodge, the speedo gear is a 17 tooth, the diff is a 3.73. Now suppose this exactly what was in the 51 dodge, even the tires are a close size. But, the speedo it self could be vastly different, now suppose the speedo cable is turning the correct speed for the 51 Dodge speedo, maybe it needs to be like this, maybe the 60 mph mark on my 47 Chrysler speedo, is the point where on the 51 Dodge it would register at 50 mph.

Just an idea, not sure if it makes sense.

[Plymouthy Adams]

would think the 3.9 is the more common rear gear to the 51 Dodge..

[55 Fargo]

Thats true, the 3.9 could be, that could explain the 17 tooth speedo gear, but 17 tooth gears could be used with a 3.73 also, I suppose, what do you think of my speedo difference in the 2 cars dissertation............oh heck, I can just adjust for the speeds, put little red stick-on dots at the 30,40 ,50 and 60 point on speedo.

Here in Canada, when they switched to metric, you could go out and buy lillte stick on numbers to place on the speedo to indicate your KMH against the MPH.

[55 Fargo]

Here is a pic of a 1951 Dodge speedo, very different than a 47 Chrysler speedo, so maybe my theory is not so out there, this is a big round speedo, like I said, possibly it would register more accurately on this speedo, with my current set-up. But displays 10 mph faster on my Chrysler speedo........Fred

[Young Ed]

Fred I hope thats an old picture cause aaaaa your steering column isn't connected!

[Jim Yergin]

Fred,

I would think that if you could get access to a Chrysler manual for your year it would have a chart like the one in the Plymouth manual for the speedo gears. Then you could check out your theory and see if the Chrysler used a different tooth count for the 3.73 rear end.

Jim Yergin

[55 Fargo]

There was nevr a 3.73 used in a Chrysler that I am aware of, they used the following 3.54,3.90,4.3. The 3.73 was used in Desoto cars.

The speedo idea, might not be so crazy, I know what I need for the change, either a 18 or 19 tooth gear, the 19 tooth gear I have is the wrong type, as it is from the semi-auto 4 spd trans, mine is a 3spd trans from 1951, the sppedo cable goes in the left side not the right side

[Phil Martin]

I was told I need about 13 teeth. Where would I find something like that?

[55 Fargo]

Just went on a test drive, the odometer is dead on mile after mile, is out a bit after 5 miles. So the speedo gear is right for the rear axle ratio, tire size, the problem must be the speedo itself, I do have a spare on, will put it in and see what happens. Am I on the right track do you think......Fred

[Normspeed]

I was told I need about 13 teeth. Where would I find something like that?

Front row of a Willie Nelson concert? Just kidding. You could try this place, they seem to have just about anything for these trannies, either new or used. They supplied all the parts I needed to rebuild my 53 3 speed/overdrive.

http://www.txchange.com/main.htm

[norrism1]

Rockwood, Speedos in the older vehicles had an adjustment to fine tune the speedo. Some had a little wheel like a windup watch and others had a hole in the side to insert a tool to make adjustments.

[55 Fargo]

Is this the case with the P15 and Chrysler C38 speedo, and how is it done................Thanx Fred

[De Soto Frank]

The odometer is a geared-coupling: it either works or it doesn't. If the odometer "gains" or "loses" over a measured mile, then either the tires or the speedo drive gear are incorrect (or combination).

The SPEEDOMETER however, is a magnetic device, with no "hard" mechanical connection between the drive cable and the needle; the cable spins a magnet inside of an aluminum cup attached to the needle; as vehicle speed increases, the field created by the whirling magnet acts upon the cup, causing the needle to move away from "0". A coiled "hairspring" ( like the one in Grandpa's pocket-watch ) returns the needle to "0", and has an adjustment for calibration (also like G-pa's watch). (This is a pretty simplified explanation, but the general principle is correct, even if I've confused a few details... the "magentic" speedo has pretty-much been the standard mechanical speedo since the mid-1920's.)

I have heard of speedo magnets losing some of their umph over the years, which usually causes the speedo to register "slow".

If the odo is registering correctly (and "no error" for four miles is pretty good), then it seems likely that your speedo needs to be recalibrated.

Most analogue gauges are designed to give their most accurate reading towards the center of their indicating scale.

Speedos can be an exception, and I remember reading a review from Tom McCahill on the 1955 Hudson Hornet Hollywood, commenting that the Hudson speedo clocked about 7 MPH "slow", as opposed to the industry norm of clocking about 7 MPH "fast".

I'm not aware of MoPar speedos being "externally adjustable"... I believe this is a "bench job", probably best left to a speedo shop.

If you have a spare cluster, try that first - (maybe clean it up internally, to brighten the lights). You could even hook the cable up to the spare for testing, before pulling the exiting cluster out of your dash...

[55 Fargo]

Thanx Frank, I like the idea of hooking up my spare speedo and testing it before mounting it. My odometer was pretty well bang on........Fred

[martybose]

It was explained to me that most mechanical speedometers are set up for a cable that turns 1000 revolutions in a mile, so at 60 MPH it would be turning 1000 RPM.

When they checked my speedometer they checked the number of turns per mile and installed a gearbox to get it to 1000, then they dialled in my speedometer on a bench with a 1000 RPM drive motor.

So far so good.

Marty