Round speedometer adjustment/calibration ?
 

My round speedo reads about 15% slow. Indicated 30 is about 36mph. I tried a search on adjusting the speedo and it seems it can be done by moving a weigh or something but I couldn't find a procedure on how to actually do this. Can anybody help ?? thanks

Re: Round speedometer adjustment/calibration ?
 

With the passage if time, the magnet in the speedo weakens, resulting in low readings. Bearings made stiff because the original lubricant has dried out also contribute. There is an adjustment in the instrument and I'm sure someone here will be able to post the appropriate diagrams. I suggest you get in touch with someone like papnic who advertises on the swap forum here and get him to service your speedo. It will come back like new - lubed and adjusted. New numbers too, if you like.

Re: Round speedometer adjustment/calibration ?
 

Move the speed wheel to the speed you are reading currently reading (44MPH) and note the position of the hair spring adjusting screw (let's say 4 to10 o'clock). Now move the speed wheel to the correctedMPH( 40) you want the speedo to read. Look at the hair spring adjusting screwand move it to the position you saw for the 44MPH reading. Your speedo should now be calibrated.

http://www.modelahouse.com/tech/spee...r/fischer.html

Re: Round speedometer adjustment/calibration ?
 

Leave the hair sping as it is. Calibration is accomplished by moving the adjusting plate on the bottom of the magnet.

The paragraph below is from the article by Gordon Fischer of THE BEAVER CHAPTER, MAFCA Portland, Oregon

http://www.modelahouse.com/tech/spee...r/fischer.html
CALIBRATION

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

Re: Round speedometer adjustment/calibration ?
 

Re: Round speedometer adjustment/calibration ?
 

Calibration is one of the time consuming parts of speedometer repair. It is much easier if you have a calibrating machine and a way to charge the magnet. Moving the lever on the magnet and adjusting the hairspring are both ways to do it, but have different effects. Basically to get it accurate over the whole driving range, you have to do both. It is simple to make it read right at any one speed, it's another to make it accurate 0 to 60 mph.

Mark

Re: Round speedometer adjustment/calibration ?
 

My experience on a couple of speedo's. I made a RPM counter with some electronics and on Bob's website (already listed) there was a chart for some speeds and rpm's.

I just moved the magnet shade and found I got it reading dead on for the most part with it like a 1 MPH off at the lower speeds. My feeling is mess with it the least, the least problems can be had.

For those who like to play, there are circuits out there for doing IR RPM monitoring. You can put that into a Arduino or just get a volt meter that does freq counting. I used a battery powered drill to do the turning.

FWIW, I believe all the speedo's read low that I tried.

Re: Round speedometer adjustment/calibration ?
 

Yes Al, that is how I adjusted mine, and it helped.

Re: Round speedometer adjustment/calibration ?
 

I checked my speedometer against my GPS and found it to read 6mph high at 20 mph indicated and as vehicle speed increased the dial moved gradually slower until at 45 mph both matched.
This is very interesting information, thank you.

Re: Round speedometer adjustment/calibration ?
 

Mine gets farther off the faster I go.