Speedometer calibration 1940-1946 Truck

H

htc

Member
Hello,

I am trying to calibrate a speedometer that was used in a 1941 Ford Truck.
As I want to do the calibration outside the car I need to know the relationship between the RPM of the speedometer cable and the miles indication.

E.g. for a Willys MB or a GPW this relation is 60 mph for 1000 rpm cable rotation (which comes from a gear in the transmission).

Is there a place where I can find this info?

Thanks
Hans
 
Hello

Thank you for the link.
Yes, probably these guys know about the transmission gear ratio to speed.

But as I plan to calibrate the speedometer myself and I am not able to send it to Rusty, I doubt that they will give me he information.

Is there not a technical manual or so that would contain this information?

Greetings
Hans
 
All you need is a variable speed drill, a way to drive the speedometer with the drill, and a clock with a second hand.
Spin the input up to so that the speedometer is showing a steady 60 miles per hour. When the tenths wheel in the odometer is showing a full number watch the clock for exactly one minute. In that time the odometer should go exactly one mile. If the odometer goes more than a mile in one minute at 60 mph then the speedometer is reading slow, reduce tension on the spring. If the odometer goes less than a mile the speedometer is reading fast, increase tension on the spring.
 
Hello 38 Coupe,

That is really smart! And indeed it can work.

You can use a drill but it is not easy to work with so
I do have a specially made motor that provides 0 to 1000 rpm. This is great for calibrating the MB/GPW speedometers. They do 60 mph at 1000 rpm.

I could use it for the Ford Truck speedometers also by working like you say, which, once again, is smart thinking.

This being said, if somebody could tell me what the speed needs to be at 1000 rpm I do not have to time for one minute and so on. Much easier.

Whatever method I use to calibrate it leaves me with the problem of (using your words) increase and decrease tension on the hairspring.
Did you ever try this?
Impossible to do for more than a 5 % change.

Unless you know some method that I do not know about :-)

Greetings
Hans
 
On our '41 our speedo was correct with stock (3.78 rear). I then changed to a 302 w/ C-4 trans and 2.75 rear. speedo was off so I made the adj. of the spring. I could get it within 2 MPH at most speeds above 20MPH. worked at speeds above 20 correctly via our GPS. I figured if speedo was wrong at 20 who cares! I pulled the speedo out to make adj. Took a couple tries.
 
htc said:
Hello

Thank you for the link.
Yes, probably these guys know about the transmission gear ratio to speed.

But as I plan to calibrate the speedometer myself and I am not able to send it to Rusty, I doubt that they will give me he information.

Is there not a technical manual or so that would contain this information?

Greetings
Hans
Click to expand...
.

Understood but I'd still contact him. He's been very helpful in all my dealings with him. He's a car guy and has offered advice to many members over on the HAMB. In fact he's an Alliance vendor, meaning he offers discounts to HAMB Alliance members. IMO it would be worth contacting him but your choice.
 
Speedometer adapters just require you to tell them what your speedo reads at a certain correct speed. I used a GPS to drive at 60 mph. I told them what the speedo read out was and they came up with the adapter.
 
hello Juergen,

I did not know about he speedometer adapters. Interesting.

However, I only have the speedometer and not the car to drive. I want to calibrated the speedometer by changing the hairspring stiffness or the magnetization of the magnet.

Greetings
Hans
 
I was recently able to change the calibration of a 1954 Ford car speedometer by 20%. A friend decided to get his parts car driveable and we swapped in a 2.75 ratio rear axle, 20% higher speed than the highest Ford offered in 1954. He took a nail and ground four flats on the end so that it would drive the speedometer, chucked it in a battery powered drill, and handed everything to me. I adjusted the speedometer spring as far as it would go and spun things up. At a steady indicated 72 mph it went one mile in one minute, 20% faster than stock. Give this a try, it is cheap and you have nothing to lose.
 
38 coupe said:
I was recently able to change the calibration of a 1954 Ford car speedometer by 20%. A friend decided to get his parts car driveable and we swapped in a 2.75 ratio rear axle, 20% higher speed than the highest Ford offered in 1954. He took a nail and ground four flats on the end so that it would drive the speedometer, chucked it in a battery powered drill, and handed everything to me. I adjusted the speedometer spring as far as it would go and spun things up. At a steady indicated 72 mph it went one mile in one minute, 20% faster than stock. Give this a try, it is cheap and you have nothing to lose.
Click to expand...
that's exactly how I did our '41 but I adjusted reinstalled speedo out of the car a couple times!
 
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