can anyone help

hi everybody , i have a small problem , my speedo is under reading, when driven on a short journey in urban traffic its fine but when im on the motorway i could be doing 65 according to sat nav but the speedo reads about 45 and gradually gets lower as the journey progresses. it also swings up to about what the speed should be when i lift off of the accelerator on overrun , i have fitted 2 new cables ( a 2 piece and a single ) and still have the problem can anybody shed some light on what may be happening with the overrun jobby and and possibly come up with an answer, thanks people. by the way series 3 2 1/4 petrol

try a different speedo. If that doesn't cure it then the cog that drives the cable at the gearbox/transfer end of the cable is worn.

you can get up to 65mph??!!! speedo has probably gone into shock and gone for a lie down!!!

Check that the nut in the middle of the handbrake (hidden by the propshaft) is tight. The speedo drive is only friction driven so if that nut isn't tight enough it allows the drive to slip.

thank so far to all of you , yes landybird my dear friend i can get 70 out of it foot to the floor but no way up a slight incline it dies a death,thanks to redhand was going to go through the process of elimination when i get back from newark but the flaying needle puzzled me and thanks to dave c , yes i had heard in the past that this could be the problem but as the nut was rammed up and had a split pin in it to secure it i thought this would be unlikely thoughi suppose wear may need it nipping up , dave how does the speedo drive work being its a friction fit what is it frictioning against in the housing , and could you explain the reason for the flaying needle on overrun if poss , thanks again everybody

No, I can't explain why the reading should go up on overrun, I was just suggesting a common cause of fluctuating needles.
I can't explain how it works either, it's a case of a picture paints a thousand words, so look in a parts book(best of all, strip one down, but probably an impractical suggestion.)

thanks for the reply dave but was thinking of what you said earlier and it makes sense about the friction fit , the nut and a larey needle , under normal load the engine will be driving the rear output shaft and the (shall we say worn) worm drive will be turning , but on overrun the engine ceases to turn the drive shaft and its the rear wheels and momentum which are providing the power to turn the shaft instead , and as you say if the" friction fit" is worn from the turning power of the engine alone it may just be possibal that power from the opposite end of the drive train ie road wheels will spin the worm drive up to speed , just a thought and will investigate for a loose rear nut thanks again people

Good thinking!

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your engines going to tilt slightly one way on drive and the other way on overrun, your speedo cable is not being trapped anywhere is it?, causing it to slip on drive when its being nipped, then be released on overrun and read normal.

hello doc , yes your reasoning also makes sense but dont think this is the problem but will check anyway thanks matey