Speedometer error

One can purchase a PATC ratio adapter from Ebay based on the percentage of error calculated. Avoids removing the speedometer. Some of the vendors' new turtle gears may not fit the 19 case without gasket or other adjustments.
 
It's amazing how different people's standards are. I can't imagine careing about 3mph. My curiosity is aroused though, as to what the adjustment mechanism is. I restored mine, but cant remember any adjustment, other than the turtle gear.
 
I have posted this adjustment on VFF and it come 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.
 
Read the section on calibration in the below article on rebuilding speedometers by Gordon Fisher.

"The odometer and trip are gear driven and no adjustment can be made. Drive a measured mile, or better yet five miles, at any speed. If the odometer and trip do not agree with the measured distance, the gear, in the gear box on the drive shaft, is probably incorrect for the ring gear, or the tires and wheels. A good speedometer shop can supply a correct gear box to make the odometer accurate. With the odometer accurate, the speed reading can be varied a few mph by moving the adjusting plate on the bottom of the magnet. Moving the plate counter clockwise (looking at the magnet from below) will make the speed read faster, move it clockwise for a slower speed-reading. A variation of from 4 to 6 mph is possible, if the magnet is strong. The tension on the hairspring will also have a substantial effect. The spring must just have tension to return the speed cup to a positive 0 every time. The most accurate way (outside of a speedometer shop) to check the speed calibration is to compare it with the speed of a modern car at the speed you usually drive, probably about 40 mph. You can also time how many seconds it takes you to drive one measured mile at any steady speed. Divide those seconds into 3600 to find the true speed. If your speedometer is accurate at a speed of 30 or 40 mph, it is as accurate at all other speeds, as it can be made. That part is built in (partially into the decal), and there is no adjustment.

The calibrating plate on the magnet (fig. 1) can be adjusted while the speedometer is completely assembled. Once you have determined that the odometer measures the correct mileage, you can calibrate the speed dial, if it is off a little. Make the tools shown in figure 13, only the 7/16" dimension must be exact. On the back of the case, remove the plug located immediately above the screw head. Insert the 7/16" end of tool #7 through
the hole, turning it up so it will contact, or very nearly contact, the bottom of the magnet. While holding it in that position (do not let it turn), turn the main shaft with the squared tool #8 Turn clockwise until you feel the calibrating plate pushing against tool #7. Hold tool #7 steady, turn tool #8 just a little bit until you feel the calibrating plate move. This will normally make the unit read two or three MPH faster. If you turn the main shaft counterclockwise, your speedometer will show up to 3 or 4 miles slower. The possible speed readings may vary, depending on the strength of the magnet, and the tension of the hair spring. Before assembling the unit into its case, hold tool #7 in the slot just above the screw hole, and observe how the tool will contact the adjusting plate. Note that the adjusting plate is slotted and will only move a certain distance.

My drills turn the following speeds and show the following MPH when running a speedometer. 465 RPM = 30 MPH, 560 RPM = 35 MPH, 650 RPM = 38 MPH and 1025 RPM = 57 MPH. These numbers were taken from working speedometers before they were disassembled and should be reasonably close. The most accurate check is by comparison with the speed of a modern car."




http://www.modelahouse.com/tech/speedometer/fischer.html
 
duke36 said:
One can purchase a PATC ratio adapter from Ebay based on the percentage of error calculated. Avoids removing the speedometer. Some of the vendors' new turtle gears may not fit the 19 case without gasket or other adjustments.
Click to expand...


Thanks for posting. I didn’t know such a device exists.


Sent from my iPhone using Tapatalk
 
Well talk about speedy repair service, Leonard received my speedometer head in the mail only this morning, and emailed me this afternoon telling me it was done and as good as new. WOW I am impressed.
Anyone needing speedometer work done I highly recommend Leonard. Here's his contact info.
https://www.fordbarn.com/forum/showthread.php?t=277422&highlight=papanick
HEY ALL REBUILT SPEEDOMETERS ON SALE WOULD LIKE TO LOWER THE STOCK. EMAIL ME AT jrnichols642@yahoo.com WHAT YOU NEED OR HAVE TO GET REBUILD. PAPANICK.
 
Mine is off too. Speedometer can say 40; School zone radar signs say one thing, my wife driving behind me says another thing and GPS says another. It's all kind of relative. Do I really care whether I am going 45 or 50, when at 45 it feels like I am going 70? I am not likely to break any speeding laws, so I just smile and drive.
 
Some states here have a speed check on the motorways. It would be a radar thing. You need to keep a constant speed as you go thru for the radar and gps. My gps speedo is dead accurate. They measure the same. Here is what the local manufacturer says:

Is the speed shown on my HillSpeed HUD 09 correct?
Where the HillSpeed HUD 09 can access sufficient GPS satellites (3 minimum), the display operates and is updated once per second. The displayed speed will be correct and accurate. Where the vehicle is accelerating quickly or braking (slowing) quickly, the displayed speed will lag behind the actual vehicle speed until the display next updates. When the vehicle is traveling at a reasonably constant speed (not accelerating sharply or braking heavily) the speed display will be accurate.
 
Those speed checks on highways are rarely working here.
The speedo in my CCPU reads 40 when GPS says 55 but if i'm doing a longer trip, it slowly becomes closer. It has been serviced and adjusted to the limit. I think too many people expect modern performance from these 90 year old cars (speedo included). I'm darned sure if I get to 90, I won't be performing as well as I did when I was new, let alone someone born into this electronic world.
 
It’s bugged me that my speedometer is off by about 10%. I have a Tremec T170 (1984 Ford Bronco 4 speed with overdrive) transmission. Today, I took a closer look at the speedometer in first, second, and third gear and compared it to GPS speed, and I was surprised that the speedometer fairly closely matched the GPS speed. It’s only in overdrive that I see the speedometer 10% faster than the GPS.


Sent from my iPhone using Tapatalk
 
Inside the 28/29 Stewart Warner speedometers there is a adjustable magnet for calibration. Just be careful not to ruin the speedometer. They don’t make them anymore.
 
My 29 reads about 10% faster than actually traveling. It has an 18 tooth gear. I may try a 19 tooth gear to see if that gets me closer.
 
You must log in or register to reply here.
Back
Top