Speedometer Calibration
 

I have a box full of spare speedometers that I have been working on.
I have got most of them working and calibrated, I have built tools to calibrate them on the bench using cable rpm's versus MPH it is adjustable and I can check all speeds for accuracy. I have one speedometer that reads 2 or 3 MPH low at 15 and 20 MPH and 2 or 3 MPH high at 60 MPH. It is pretty close at 40 and 45 MPH. I have been playing with the magnet and spring but cannot get it
accurate at all speeds. I believe that it is a spring issue, I have loosened and tightened the spring slightly without any improvement, do I need to make larger adjustments on the spring or does the spring need replacement.
Do not tell me to send it to someone, I enjoy learning and doing all my own work, and these are spare speedometers.

Re: Speedometer Calibration
 

I am wondering if the pivot point is dirty just at one spot in its travel near the low end.

Re: Speedometer Calibration
 

Any chance of doing a Speedo 101 class? I have one which runs 5mph slow at 40mph ( cost me £90 to find that out when I passed a camera!). I'd like to know how to fix it without sending it away.

Re: Speedometer Calibration
 

I had my speedometer professionally rebuilt when I restored the car 2,000 miles ago in 1998 or so. Driving it seemed to read high. These days with GPS apps like WAZE I'd say I'm reading 5-7 mph high at 40.

If it's field adjustable I'd give it a try also, but I'm only going to get a ticket in a 25mph zone, if ever.

Re: Speedometer Calibration
 

I am not a speedometer repairman, just a hobby guy that likes to do most of my own work instead of sending stuff out and paying other people to do my hobby.
I am not a speedometer expert I am trying to learn enough to repair my own speedometers.
They can be calibrated on the bench, but the tools to do it would cost much more than buying several rebuilt units.
you must have a variable speed motor and a accurate RPM meter for the cable.
A variable speed drill is not fast enough and the speed will not hold steady.
The pictures show some of the equipment needed, some items will need to be custom made in a machine shop. Then there are the tools to recharge the magnet, and on and on.

Re: Speedometer Calibration
 

I have posted this adjustment on VFF and it comes directly from Leonard Nickols who rebuilds and sells Model A speedometers. This is an adjust to correct the speed error on the round Stewart Warner speedometer. When rebuilders rebuild speedometers they mostly check to see that the mileage(odometer) reads correctly, not the speed.........

Remove the speedometer from the dashboard. Now carefully remove the cover on the back of the speedometer. You will see a very small hair spring on the top of the rotating speed wheel which has an small screw at it's center. Now move the speed wheel to show the speed you were reading while driving (50MPH) and note the position of the hair spring adjusting screw (let's say the slot runs 4 to10 o'clock). Now move the speed wheel to the corrected MPH you want the speedo to read (46-GPS). Look at the hair spring adjusting screw and adjust it to the position you saw for the 50 MPH reading. Your speedo should now be calibrated.

Re: Speedometer Calibration
 

That is true for a lot of speedometers but some of them are reading low at slow speeds and high at fast speeds, or they are not liner. That is what I am trying to learn how to correct.
If they are reading high or low at all speeds, that is easy to correct.

Re: Speedometer Calibration
 

Getting a speedometer to report accurately over the whole range is a challenge. I have done a couple thousand, some are a lot easier than others. It is quite common for them to be right on at 20 mph, and at 60 mph, but a couple mph high at 40 mph. I have not figured that one out yet, though it may be the hairspring rubbing on itself at different speeds.

I have a Stewart Warner "Speed Analyzer" circa 1940's that I have calibrated to check my repaired and restored units on. Some I can get in 2 minutes, some take a 1/2 hour to get into spec. Below 20 mph, they are very hard to get accurate, ditto 65 plus. I concentrate the accuracy in the 20-60 mph range. About 10% of the units I see require the magnets to be recharged, as they will not get up to speed with the adjustment at maximum. You can't just take tension off the spring cause it won't return to zero.

I had one unit that gave me fits, couldn't get to calibrate. I noticed that there was oil on the hairspring, the viscosity between the coils was enough drag to make it inaccurate. Reading low on the way up, high on the way down.

Calibration of 90+ year old instruments is a lot of fun with the proper equipment, a real challenge with out.

Re: Speedometer Calibration
 

Thanks for the information.
I can charge the magnets, they usually jump up 10 or 15 MPH after charging, I can also remove some or all of the charge with coils, but have never had to do that.
If the spring has been replaced with a reproduction spring or one from another speedometer could that cause a non lenier problem

Re: Speedometer Calibration
 

Kept playing with it and Googleing and it seems that spring tension affects the slow speeds more and the magnetism affects the fast speeds more.
With that in mind I was able to get it calibrated to + or - 1/2 MPH at all speeds from 10 to 70 MPH Thanks to everyone for your help.

Re: Speedometer Calibration
 

I repair clocks for a living. Dirty hair springs and its pivots plays a huge difference. The coils touching together by dirt or oil will cause havoc. I overhauled mine using an ultrasonic cleaner.

Re: Speedometer Calibration
 

I don't believe it is possible to set up the speedometer to be accurate over the full speed range. When a spring is used to establish a desired output value on any device, you have what is referred to as "proportional band offset." In other words, you can make the speedometer repeatable and accurate for a given speed but at other speeds the indicated value will be slightly higher or lower than the calibrated setting. This is because the spring isn't perfect, its spring rate (force vs. displacement) isn't linear. I would set up the speedometer to read 40 mph at 40 mph and just know that your indicated speed will be higher at lower speeds and lower at higher speeds. I hope I didn't get too windy on this post!! Ed

Re: Speedometer Calibration
 

Re: Speedometer Calibration
 

Are you speaking from actual hands on experience from calibrating speedometers, or theory.
Yes springs are not linear, however the effect of the magnetic field from the rotating magnet on the cup also is not linear, with practice you can learn to balance out these forces and make it linear. when I calibrate speedometers I adjust the spring and the magnet and can usually get a linear reading from 20 to 60 MPH plus or minus 1 MPH. It is not east to do and takes some time but it works most of the time.

Re: Speedometer Calibration
 

Whenever I worked on speedometers I was happy just to get them working, never worried about how accurate they were.