elctronic speedo wavering..

rrc running standard 1993 lr transducer and speedo cable..speedo "wavers" between 40-50 mph....was running standard speedo..but as a two door have fitted oe pod back in with brand new vdo speedo...(in place of dead mechanical one,was using later pod from 93,but looks pooh so now gone)

WITH lr speedo its fine..with vdo it wavers...etb gauges have no idea..have filtered,altered amplitude etc of signal all to no avail..i would have though cable,but cable is perfect and besides is perfect with lr speedo..

and no i cant fit the 1993 lr speedo in the 2 door pod..no room

signal looks a1 on scope..cable is good..have swapped out transducer

stumped...ideas?

oh..i should have said..its fine upo to 40mph then wanders about + or - 5to 10 mph untill it hits 50 when it settles down again..

Have you checked / swapped the speedo head?
From what you're saying it's not the input signal so you're not leaving much.
Even if it was a spurious signal about the appropriate frequency then that should show up on the scope or have been eliminated by filtering.