Repairing Jaeger and Smiths Speedometers
An Article by Anthony Rhodes Copyright 2000
Another option you have is to have an adapter made. It will have two gears with a certain number of teeth to convert a certain input cable R.P.M. into a certain output R.P.M. For instance, if I need a 1312 and I have a 1184 speedometer. They would probably make an adapter with 41 teeth on the input gear and 37 teeth on the output wheel. One manufacturer of these is APT Instruments in Bloomington, MN (612-881-7095). The cost for one is reported to be about $40. I spoke with them about the construction of an adapter. It seems that there is difficulty due to the difference between English drive cables and US types. They made it sound rather complicated, and I did not spend the time (I was paying long distance charges) to resolve the details of the difficulty. They suggested that the speedometer "head" be re calibrated. I am not sure that the guy I talked to understood the exact construction of a vintage Smiths/Jaeger speedometer. He did say that if I were to give him the true miles and odometer miles reading they would be calibrate it perfectly for $120, and for that price, throw in a cleaning too! They would also then check, calibrate, and if necessary repair speedometer as well as to calibrate the odometer. With an adapter, you do not need to look for scarce or impossible speedometer calibrations and still keep your original equipment completely original.
Editors Note: Things are a little different here, for the Sunbeam Tiger. There are two things to keep in mind. Firstly, unless someone has gotten inside your factory speedometer, the calibration is stamped on the face of the instrument.
There is a number below the odometer, on the right side. In this case it is 980. This IS the calibration. . The Tiger Mk I unit pictured is set to 980 turns per mile. Some units are set at 1050, or 1725 turns per mile. US speedometers are universally calibrated at 1000 cable turns per mile traveled. All transmission and speedometer cable gearing is chosen for the vehicle rear end ratio and tire size design to make this so. The British, on the other hand, seem to like to make the speedometer take the burden of adjusting for these factory chosen features. Neither system will be accurate if the tires, rear end ratio, cable gear, or internal transmission gears have been changed. So you know that the adapter you will need MUST provide the indicate calibration, to adjust for any changes in those items. These adapter units are not intended to "calibrate" a faulty speedometer. In any case the original calibration is neither scarce nor impossible to find, it is in front of your nose. Mk II's had a different number, as well as some Alpines, including those with the overdrive.
The second important Tiger Only feature is that the speedometer cable inner core and outer cable and couplings are NOT "standard" English, but FORD US standard design. The adapters can be purchased with the correct fittings. Cables and replacement cores can be made by any US speedometer shop, as their tools, fittings, and cores fit.
These APT adapters are about 1 inch by 2 inches cross section, and about 2 to 3 inches long, when I used them 25 years ago. It is unlikely that you would mount them to the speedometer case for lack of room or strength of the speedometer case structure. I mounted mine in a convenient location alone the original speedometer cable run, preferably accessible and protected from heat or the elements, and had custom length cables made to connect it to the transmission and speedometer. The APT case top can be removed, and the gears easily changed, in the event of a change to your tires, rear end, transmission, etc.
HOWEVER, Tony is absolutely correct that, even if the original factory calibration can be positively known, the speedometer's actual condition my require the re-build of worn components, and the re-magnetization and calibration of the unit. We already know of sources for rebuilding these units. Finding original gears, to change the original odometer calibration can be difficult, but the speedometer readings are adjustable by magnetization and spring tension. Do not expect accuracy over a large speed range. They are usually set to be most accurate at highway speed limits, and taper off from there in both directions.
Of course, during the calibration tests you need to be using the tires you plan on keeping on the car and they need to be properly inflated. As the tires wear, the speedometer calibration will vary. It is not worth getting too picky about the exact accuracy of the calibration since tires of the same nominal size may differ by many tenths of a percent in their "turns-per-mile".
Remove the works from the speedometer as described in the prior section. Turn the input shaft by hand. You should be able to twist the magnet wheel easily. As you turn the wheel, it should drive one or two gears from a worm. As each gear turns it should move a pawl via an eccentric pivot. The pawl should be seen to advance the 1/10's wheel of the odometer one tooth for each pull.
To further test the odometer wheels, you will need to be able to spin the works at a reasonable speed. The only way to do this without crashing your car is to use a spare tachometer or speedometer cable and a drill set to reverse. Run the drill about 1000 r.p.m. As the drill turns the cable, you should see the worm gears turning and the pawls advancing the 1/10's wheel every few seconds. As the 1/10's passes 9, you should see the miles wheel advance by one.
If the gears and pawls are moving the wheels correctly, but the wheels fail to advance, then there is a problem with the wheels themselves. The best correction is to swap the entire wheel sets with a different speedometer of the same calibration. If that is not possible, then the wheels themselves may be changed by dismantling the wheel sets. You will need a wheel set from a similar speedometer, though the calibration will not matter.