Speedo drive gear

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kyamon

kyamon

Active Member
Posts
114
Location
Lausanne, Switzerland
Hello

My speedo reads about 20% low and I am wondering what can be done about this - it looks like I have the metal-type gear (FRC1536), and I have not found this to be sold with different numbers of teeth, so either it works or it does not. Is that correct? Or can the plastic ones be used instead of my current one (with the correct mounting, of course)?
The speedo reads quite stable (new cable installed), but consistently low. This is no problem for me since I know how to correct, and I can always use my gps to check, but the Swiss equivalent of the MOT requires speedometer readings to be more precise than this.

Thanks!
 
I have 235/85, for all I know... I was thinking the same, but I dont know if there is a modification with the speedo drive to make it work with smaller wheels, nor why they would have installed that. (It is a 1974 6-cyl, btw)
 
kyamon said:
I have 235/85, for all I know... I was thinking the same, but I dont know if there is a modification with the speedo drive to make it work with smaller wheels, nor why they would have installed that. (It is a 1974 6-cyl, btw)
Click to expand...
theres no land rover option ,i believe you can get gauges recalibrated
 
Do a search as there are some good websites explaining it. If you have larger tyres or different diffs you need to recalculate. Its all done by turns per mile and the Smiths speedo heads have the turns per mile on the face - its around 1500 for the smaller tyre and 1400 for the larger and 1300 ish for the higher diffs. You can get the speedos adjusted once you have worked out the turns per mile. Use the rolling radius which is slightly smaller then the tyre OD. You can get some in-line gear boxes with chageable gears but they are fiedishly expensive. Turn up on e-bay US once in a blue moon and I've yet to find one. The hot-rod builders need them as they change wheel sizes. One of my "get round to it sometime" jobs is to strip a Herald speedo that's around 1250 and I recon about right for my set up and fit the key parts int0 my S2A speedo head.
https://www.marks4wd.com/speedo-correction.html
 
Thanks everybody for your replies and suggestions. I drove about 100 km today and have the impression that the odometer reads correctly - at least what it read was not too much off from the prediction by the gps (but I did not measure the actual distance I drove, so it may also be that the prediction was inaccurate), certainly much less than the 20% error on the speedo. So it may indeed be that the speedo itself is off, but at least at some point the head was intended for the current wheel size.
Not knowing how the speedo actually functions I dont know if this can be repaired. The head has an electrical connection if I am not mistaken - what is this for? Could it be that the voltage stabilisation is off, and that the speedo gets a wrong voltage, for example?
 
>>Could it be that the voltage stabilisation is off, and that the speedo gets a wrong voltage, for example?

No, the voltage regulator is only for the fuel and temperature gauges.

The speedometer and the mileage counter are not electrically powered. The speedo reading is determined by the rate of rotation of the speedo cable and the calibration of the speedo head; there's nothing else involved.

The back of the speedo was simply a convenient point to fasten the voltage regulator.
 
Tall Ratbag said:
>>Could it be that the voltage stabilisation is off, and that the speedo gets a wrong voltage, for example?

No, the voltage regulator is only for the fuel and temperature gauges.

The speedometer and the mileage counter are not electrically powered. The speedo reading is determined by the rate of rotation of the speedo cable and the calibration of the speedo head; there's nothing else involved.

The back of the speedo was simply a convenient point to fasten the voltage regulator.
Click to expand...

OK, thanks. I will try the recalibration method described in the document you posted - that looks fairly straightforward.
 
kyamon said:
OK, thanks. I will try the recalibration method described in the document you posted - that looks fairly straightforward.
Click to expand...

If you haven't worked on speedometers (or similar fiddly and delicate mechanisms) before, it's probably worth obtaining a cheap second hand unit to practice on.

For me, this strategy worked well because I found that the fault in my speedo head [non working odometer] could be repaired by using a gear wheel from the second unit I had bought to practice on.
 
Advice to practice on an old unit is very good advice! Be very careful if you use any cleaner as some (I found out the hard way) strip the numbers off. To get it to read higher you have to lengthen the hair spring which is not easy as its clamped in place so they can calibrate it (good) but they put a drop of solder on to lock it (bad) so you've got to get that solder off before you can pull some more spring through. Run it on a drill and note the reading, then make adjustment to get the right % correctrion. This is far from exact as you are only setting a curve at 2 points, zero and one speed but if you can get the the "drill speed" around 30 mph/ 50kph you should be reasonbly accurate at the key speed limits. From experience I can say this goes only one of 2 ways: a) an hour later you have a corrected speedo, or b) 6 hours later all the bits are in the bin and you are on e-bay shopping for a new speedo, its a gamble.
 
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