When the Smiths 8000 rpm RVI tacho was introduced for the Lotus 7 in 1962, the original speedometer looked a little outdated in comparison. We wanted to find a contemporary matching speedometer and we found one on ebay.
Matching tachometer and speedometer
While this speedometer is basically contemporary Smiths, the dial is probably not (the calibration information is missing). The instrument showed the correct mph, but the odometer counted twice the number of miles. Encouraged by Anthony Rhodes “Repairing Jaeger & Smiths Speedometers” (Google if you want it) we decided to open the box to see if we could mend it. Here we found Anthony Rhodes’ description invaluable when we had to refit the small springs and spacers in the right sequence.
Odometer wheels disassembled
The worm gear (20 teeth) and the drive gear (48 teeth) should give a calibration of 960 revs per mile, but we measured half of that on the odometer. We soon discovered that the outermost wheel (at the arrow on the picture, referred to as the “1/10th wheel” by Rhodes) had two “cogs” insted of one. This in turn caused the “one mile wheel” to turn a digit for every half revolution of the “1/10th wheel”, effectively halfing the calibration. This was corrected by filing off one of the “cogs”. The now 960 rpm calibration is low by 6% which we find acceptable.
Coincidentally, we had another (newer Smiths) speedometer which exhibited exactly the same fault, counting double miles. In this case the cause was a different one. We noticed that the pawl advanced the driving gear by two teeth instead of one, effectively halfing the number of teeth on the gear.
Pawl and odometer driving gear
The pawl movement is accomplished by a worm gear driving an eccentric axle. It was evident that the pawl movement was too large. Hence, we found an axle with less eccentricity which solved the problem.
Axles with different eccentricities.