Repairing Jaeger and Smiths Speedometers
An Article by Anthony Rhodes Copyright 2000
Page 7
As an example, you want to set your speedometer to 1152 calibration. You need to calculate what MPH should be indicated when you run the drill at its maximum speed. The MPH will be 60 * RPM / 1152. So, in this case the MPH = 60 * 1200 / 1152 = 62.5. Now turn on the drill and look at the indicated speed on the speedometer. Turn off the drill and hold the disk with the pointer at the observed speed. Gently push the pointer to the calculated speed (62.5) and retest. Adjust the pointer until it reads exactly the calculated speed. Now you can attach the speedometer to the cable in the car and drive a measured mile and adjust as outlined above. These adjustments will set the speedometer only. It will not set the odometer, which is gear driven as stated below.
THE ODOMETER: Mechanical Description
The odometer is gear driven. The cable turns a worm screw which turns a 32 (sometimes 20 or 25) tooth gear. This gear is directly attached to a pawl via an eccentric pivot. Every turn of the gear will pull the pawl once. The pawl turns a gear at the end of the odometer wheels. This gear has a different number of teeth depending on the calibration of the odometer. This calibration is written in small letters on the dial face above the "MPH". 1152 and 1184 are the most common on the TR4 series, Other speedometers may use a 20 or 25 tooth gear and more teeth on the odometer wheel gear. Fewer teeth on the worm gear makes the movement of the odometer wheels much smoother. The calibration is always a whole multiple of the 32 (or 20,25) tooth gear. For instance, 1152 is 32*36.
There are a few places where problems can occur.
- The worm may not turn, and this is the same problem as #1 in the speedometer section.
- The plastic 32 tooth gear can be stripped, or the clip that holds the pawl to the eccentric pivot may have fallen off allowing the pawl to fall from the eccentric.
- The spring pulling the pawl to the gear on the wheels may be weak or missing. This may prevent the pawl from touching the gear and thereby prevent any motion of the wheels.
- There is the very unlikely possibility that the wheels themselves no longer index properly.
ODOMETER: Calibration
Calibration of the odometer is not as simple as making an adjustment in the works somewhere. Being completely gear driven, you need to replace the gear on the odometer wheel axle that is moved by the pawl. To do this you need the appropriate gear from an otherwise identical speedometer with the proper calibration. The simplest method to do this is to determine what calibration you need. This can be calculated by driving a measured distance of road. The longer the better, ten miles minimum. Then compare the actual mileage to the indicated mileage. The calibration you will need will be: Old Calibration * Indicated Miles / Actual Miles. If you drive 20 miles and read your odometer to about 1/2 of a tenth, then you will get your correction factor to better than a half a percent and probably close to a quarter of a percent. This is certainly a better calibration than the car had originally.
You can calculate the THEORETICAL calibration you need by finding the "turns- per-mile" (TPM) specification of your tires. Your driveshaft RPM (at 60 mph) is TPM * Differential Ratio. The differential for the TR2-4 series is usually 3.7. It will vary in other cars. You also need to know the number of driveshaft turns per cable turn. On the TR2 through early 6 series it is 2.5 drive shaft turns to 1 cable turn (possibly 3.5 for Spits). For a TR2-4, the speedometer calibration you will need is calculated by TPM * 3.7/2.5. This is a very theoretical number and you are much better off by driving a distance on a marked highway (at any speed) and comparing the indicated milage to the real mileage.
Unless you are very lucky, you will not be able to find a speedometer with exactly the required calibration. You can calculate the closest POSSIBLE (but not necessarily available) calibration by dividing the calibration you think you need by 32 (or 20, or whatever number of teeth had by the worm gear) then rounding to the nearest whole number. For instance, if you find that your speedometer reads 11 miles when you drove 10, and the calibration is 1184, then you need a new calibration of 1184*11/10 = 1302. When divided by 32, this is 40.7. This rounds to 41, which tells us that the closest possible calibration is 32*41= 1312. You need to look for a real speedometer with a calibration of 1312, or at least as close as possible to the calculated 1302. Any Smiths or Jaeger speedometer of the same vintage ought to be very similar in the works and may be able to be swapped. For example, I needed a 1280 speedometer and found one from an MGB of the 70's vintage. It would have been a perfect match for a TR6 speedometer and was a serviceable match for a TR4 speedometer. One from the 1960's probably would have been a perfect match. I will keep looking. After I did a 30 mile odometer calibration trip, I found that I need a calibration of 1344. This just happens to be a whole multiple of 32 (32*42), and it might be possible to find an odometer gear to give the exact calibration. Between all the cars that used these basic styles of works, there is a wide variety of calibrations to be found at fleamarkets. You should keep looking, but if you find a unit that has an odometer gear that is within one tooth of your "ideal" calibration (around 2.5% variance), it will probably be quite sufficient.
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