speedo accuracy??

[mrmorrisminor]

I have noticed that the speedo in my standard '67 saloon has gradually become quite in accurate. I knew it was slightly out by following friends etc and when I first got a sat nav this confirmed it, reading 53-4 when doing 50. However, over the past 2 years or so it has progressively got worse, I now have to do 60 on the speedo to do 50 over the ground. I have a rev counter and this also confirms things as I usually cruise at 3200 on a roads which gives about 54 over the ground and used to show 58 ish on the speedo but the same revs now shows almost 65 on the speedo! The needle tends to fluctuate around quite a bit but most of the moggies i've seen seem to do this.... The tyres are the same and nothing else has altered (I've owned the car 13 years and have kept everything standard). I've also checked the cable and all seems free. I thought it might just be just how they go with age and wear as I know we're on our 3rd go round the clock but I have just this week put a '68 saloon back on the road and I was surprised to see how accurate the speedo is in that. So...... is there anything I can do other than take it to be recalibrated?

Ideas welcomed.......

[DanRodd]

ive just had a go at sorting this myself,my original speedo with 95k on it would over read by around 5-7mph.my ideal is 3mph fast,so i can drive with needle on the appropriate limit and be a few mph within the law.
i tried another speedo of the same calibration that had seen less use,and brought it down to 27mph at indicated 30,37 at 40,46 at 50 etc.They do as a rule get worse as the speed goes up.
Usually the cable will cause either the needle to dance about or go up with clicks and then bounce,i would still advise fitting a NEW cable,even if the old one seems ok,then trying another speedo.If over reading by a small and consistent amount,you can take the face off and by holding the mechanism still adjust the needle to suit.
hope this helps

[bmcecosse]

The return spring weakens with age - and so the indicated speed goes up. You could try taking the pointer off - tensioning the spring slightly and replacing it - - but the risk of doing harm is quite high!

[mrmorrisminor]

Thanks for the suggestions. A new cable sounds like it might settle things down a bit. I think as long as I know how far it is out I'll stick with it........ I'm a bit worried about ruining a working speedo with my slightly heavy hand! I'm assuming that the miles will still be right as these work on gears.