Speedometer - Mystery

J

John Larkin

Member
I recently installed a used Australian km/hour speedometer in my RHD UK specification 1974 Rover P6B automatic. Today I discovered that the odometer is OVER-reading distance by 16.8% and the speedometer is UNDER-reading speed by 14.4% (i.e. 0.856 of actual distance). There is a relationship between these two errors:- +16.8% = 1.168 times actual distance, and 0.861 = 1/1.168.

I have always thought that if your speedo over-reads then your odometer also over-reads by the same amount, and vice-versa for under-reading. My two errors seem to be inverse ratios.

I'm flummoxed!

Has anybody got an explanation? PLEASE!!!

John Larkin
 
I'm flummoxed by your post. Maths was never one of my strong points, especially after 4 pints. Put a sticky speedo cable in & estimate your speed by bisecting the arc of the speedo needle (or the progress of the red bar). It's hard enough keeping a modest classic fleet on the road without having to pick a calculator up......... :D
 
Sounds like you have a duff Speedo, or it needs a good oil up, I have just rebuilt the strip Speedo on my 2000sc as this wasn't working at all, traced it back to a broken cable, fitted a new one and that didn't solve it, then found the angle drive on the back of the Speedo was seized solid, and the arm that moves the miles round inside was very tight, a quick strip out of the angle showed it to be dry but intact, so a good dollop of molly grease got it sorted, and a drop of 3 in 1 on the arm freed that :D
 
Got to agree about the angle drive. My speedo worked up to 20mph then gave up, returning to zero and clicking. Cleaning the angle drive didn't work for me but a replacement wasn't dear and it now works fine.
 
The odometer works by gears so the speedo drive itself must be turning too fast .The speedo needle works by magnetic drag which gets weaker over a period of time so the speedo needle doesn't get pulled so far round the scale and the speed shown is less than actual
I'm sure it could be recalibrated by specialists
Dave
 
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