Repairing Jaeger and Smiths Speedometers
An Article by Anthony Rhodes Copyright 2000
Page 6
Replace the face and pointer and reindex the pointer to the proper location based on the scratch made in the beginning. If necessary calibrate the speedometer as listed below.
SPEEDOMETER: Calibration
One very observant person pointed out to me that there is a small dot at about -5 mph on the dial (on tachometers too). He found that when he pulled out the stop post on which the pointer rests when reading "zero", the pointer came to balance pointing at the dot. MG speedometers have a line in about the same position. If you are restoring a speedometer without appreciably changing its calibration, then you can probably use that dot to set the speedometer. Unless the hair spring (return spring) or the magnet have lost strength over the years, then that dot ought to provide a fairly accurate initial calibration. If you are recalibrating the speedometer, then that dot will not be accurate at all and you will have to use the calibration technique outlined above.
One note, if you are recalibrating the speedometer to a setting considerably different from the original calibration, there will probably be some error particularly at the low speeds. At zero the pointer will have significantly more or less tension on the needle against the post. This may result in considerably more or less tendency for the pointer to move at low speeds resulting in low speed error. You can see that this is already an issue for the speedometer by looking at thes noticeably smaller space between 0 and 10 than between 10 and 20. This is due to the rest pressure of the hair spring on the pointer. At zero mph, there is NOT zero force on the pointer, forcing the magnet to have to turn at some speed just to get the pointer off the rest post. (about 5mph). Due to the error you might experience, it is best to calibrate the pointer to the speed you are most concerned about, such as the national speed limit. When calibrated to give no error at a certain speed, that is the ONE speed you can be positive about. Speeds close to that will have minimal error as well.
Driving Calibration
Calibration of your speedometer is easy. If you have not serviced it, it ought to be close to the correct speed unless you have installed tires of a significantly different rolling diameter. To fine-tune your calibration, you need to drive a measured mile at exactly 60 mph indicated on the dial (you may use any other speed as well, but 60 is a nice mid-range number to use). Use a stopwatch to time your mile trip. Your actual speed is 3600 divided by the number of seconds it took to drive the mile. Now remove the speedometer from the dash, and remove the works from the case. Do not remove the pointer or face. Move the pointer to the indicated speed you used in the mile trip (60mph). This is done by gently holding the drag cup in the works. Double check that the pointer is still at the indicated speed. Now gently push the pointer to the calculated speed while being sure that the disk does not move. Now pull up some slack speedometer cable into the dash and reconnect the works of the speedometer without reinstalling the case. Carefully perch the speedometer in the dash opening without allowing the dash to contact any moving parts. Time another measured mile at exactly 60. If the time is not very close to 60 seconds readjust the speedometer. You should be getting very close to perfectly calibrated with one or two resettings of the pointer.
Bench Calibration
If your speedometer has been serviced, you can do a bench calibration quite easily. You need the speedometer, a tachometer, a speedometer or tachometer cable, and a drill that will turn about 1000 to 2000 rpm. First you need to find out exactly how fast your drill turns. Attach the drill to the drive end of the cable and attach the tachometer to the other end. Turn on the drill to maximum RPM. Watch the pointer of the tachometer. It should be quite steady. If it wavers significantly, either the drill does not turn a constant speed or the cable is binding. You can lube the cable by removing the center wire and greasing it lightly with lithium grease. Once you achieve a fairly constant reading, you can determine your drill RPM by dividing the reading by two (the ratio is indicated on the tachometer dial face: "2-1"). For instance my drill showed exactly 2400 rpm, so it was turning exactly 1200. This happens to be the maximum speed listed on its label.
Now you need to calculate what the speedometer OUGHT to be reading at that RPM. You need to know one of two things. One is the calibration number printer on the dial face of the speedometer just above the "MPH". This is the cable RPM that it takes to indicate 60 MPH. If you know that your tires are substantially different from those originally supplied with the car, you can not use that number as your final setting. If you DO know the engine RPM that corresponds with 60 MPH, then divide that by 2.5 (known correct for the TR2-6 series, other transmissions may differ. I suspect that Spitfires used 3.5) to determine the cable RPM that corresponds to 60 MPH. If you are not sure of this data, then set the speedometer to the printed calibration and then do the driving calibration discussed above.
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