Speedo reading fluctuating - likely cause?

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From Tony Lee:

Up to about 80km/h the speedo reading behaves normally (although I suspect it was never adjusted when the 19.5 wheels went on so reads lower than GPS speed), from 80 to 100 it fluctuates and even drops below 80 at times and over 100 it is steady.
I've adjusted the sensor in a little closer but haven't had a chance to road test it yet.
Anyone else had problems, and was it the sensor or the speedo in your case?
May 10, 2010 at 11:25 PM Flag Quote & Reply

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From Peter_n_Margaret:

Adjusting the sensor will probably fix it. Just don't get it toooo close....
There are dip switches on the back of the instrument for adjustment too Tony.
My speedo is spot on, but the odometer is 6% low.
Cheers,
Peter
May 10, 2010 at 11:26 PM Flag Quote & Reply

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From Tony Lee:

Adjusting the sensor closer fixed the problem - reading now nice and steady.
Just have to set the speedo dip switches for the tyres. THEN I'll be able to get some accurate fuel consumption figures.
May 10, 2010 at 11:27 PM Flag Quote & Reply

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From Tony Lee:

Anyone know the mathematics of the DIP switches. 8-bit binary number, two-bit BCD or what. And is it a divisor or a multiplier.
Tried the setting given here on the site but they are wildly in error - and anyway, I prefer speed to be spot on at 100kph so it would be good to be able to fine adjust it using ratios.
May 10, 2010 at 11:29 PM Flag Quote & Reply

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From Tony Lee:

Ah, I see there are two articles on speed settings on the site.
This one
OKA Speedo Settings
These are only a guide only (calculated by mathematics).
Tyre Size 11.00 R16 Speedo Setting 4&8 On
Tyre Size 9.00 R16 Speedo Setting 8 On
Tyre Size 305/70 R19.5 Speedo Setting 8 Off
Tyre Size 255/100R16 Speedo Setting 1& 8 Off
Tyre Size 285/70R19.5 Speedo Setting 1,2 & 8 Off
Tyre Sizes 8.25R16 and 265/70 R19.5 Speedo Setting 2,3 & 8 Off
This information supplied courtesy Peter Davis.
was the one I saw and following it produced some crazy results - possible because of a misinterpretation of what "8 OFF" means.
The other article which shows all the settings always has 8 ON so the suggestions by Peter may have the switch upside down or back to front or whatever.
May 10, 2010 at 11:30 PM